I Suggest you inspect that card slot and follow earlier advice and carefully use compressed air to flush out any possible foreign material such as fluff from the slot. If that fails to work then a trip to a Nikon repair centre is probably warranted.

I have a Pixel Pro model TW-283 wireless remote control that I use with my D7200 and D500 cameras. There are a number of YouTube videos that show how to set it up and use it. The device can be set up for single shot shooting, continuous shooting, BULB shooting, Delay shooting and Timer Schedule Shooting. I was able to later purchase a separate cable to work with my D7200 as I originally bought the device to interface with my D500.
The Pixel TW-283 comes packages with a reasonable user manual. There are two battery driven parts to the kit each part holding 2 x AAA batteries. One is the receiver which can be mounted on a flash holder shoe and the other is the transmitter which you hold after configuring it to the desired firing mode. The frequency band used between the transmitter and receiver can be changed to remove the likelihood of interference from other units operating in the near vicinity by other photographers. The mounting foot of the receiver has a 1/4" socket recessed into it increasing its mounting versatility.

Look at purchasing Steve Perry's excellent book on "Secrets To Exposure And Metering For Nikon" found at https://bcgwebstore.com/product/secrets-to-exposure-and-metering-for-nikon/

You didn't say what make and model camera you are using to take the time lapse sequence of photos.
As a matter of interest I do use my Nikon D7200 to take a series of time lapse photos. At the end of the series of photos the camera automatically assembles them into a movie which can be played back on the camera and the mov file stored on the card in the second card slot can subsequently be copied ones computer. Just recently I created a time lapse of the light show projected onto the sails of the Sydney Opera House which featured as part of the VIVID light festival in Sydney over the past fortnight.
I set the camera to take 1 shot every second for 20 minutes. That amounts to 1200 shots. After the last shot fired off, the camera processed the 1200 photos into a mov file in a matter of seconds. The movie when played takes 48 seconds to play back. It is most important to have the camera mounted very securely so that movement between shots is minimised as much as possible if you want quality output.

As an aside but importantly related to this topic - it's one thing to backup to whatever your backup destination device is, whether local or in the cloud. How many folk actually check to see whether they can actually recover their data by restoring it to another drive on their PC/laptop and checking that it all restored correctly. It may require you to purchase a spare drive at least the size of the one you are backing up and then restoring to that new device. Many people do their backups on a regular basis but have never checked to see whether the data (or image of their working drive) can be restored easily and without any grief. Sooner or later Murphy is going to catch you out if you haven't gone down that path and have clear instructions on how to go about the recovery process and have proven them to work.
Put yourself in the position where something catastrophically happened to your desktop/laptop. Worst case -it's stolen or failed beyond repair. You have to buy a new device - nothing salvageable - now what? How do you get back your system and data to how things were prior to this catastrophic event. Think of all the hours you spent over configuring the device to behave the way you wanted it to including all the apps on it. Can you remember all the steps you took?
Can your data be restored to totally different hardware? This may well be the situation you face having to buy new hardware.
All this needs to be checked out when choosing a backup app.
If you don't know how to restore your data then you are living in la-la land which will ultimately give you a lot of grief when reality kicks in..

It is a good idea to also check that you don't have any "stuck" pixels. I found this to be the case recently with my D7200. Stuck pixels are those that no longer respond to light falling on them. In my case the pixels appeared to be white. I noticed this when enlarging a NEF file in my editor. In my case I put the lens cover on the lens and snapped an image in manual mode. It was quite easy to magnify the image and then look at the rear screen whilst panning across the image to verify that a white dot was present.

I took the camera to an authorised Nikon repair centre where they were able to map out the offending pixel and clean the camera innards at the same time.

What you say is true. However, the benefits of using DX mode are at least twofold.
1. Photo composition through the lens remains as before
2. No need to spend time cropping which is detrimental to JPG images if RAW images were not taken.

I misread your question - you have an existing 300mm lens. The effective focal length in DX mode would be 450mm which would meet your requirements without costing you a nickel.

Just thinking a bit laterally - you are already familiar with your existing 80-400 lens. If you don't need the full resolution of your camera's sensor then you could operate in DX mode thereby giving an effective increase in the focal length of the lens by a factor of 1.5

Thus your lens would appear to operate as a 120 to 600mm lens.

The cameras in the latest mobile phones (cell phones for you Yankees) are quite amazing but the phones' grip for taking photos leaves a lot to be desired compared to those specialised devices designed to only take photos. Hah!!

You might like to explore specialised applications from Ashampoo.com
Here is a link to their website
https://www.ashampoo.com/en-us/partner-software/category
Look for the application called Franzis CutOut 9 Professional. It might be what you need.

I am astounded that Nikon support cannot give you a straightforward answer without having to send the camera in to them. I would imagine if the camera is new then the registration process would either accept or reject the serial number you supply.
I'm in Australia so all I have to do is supply the camera model, serial number and store from which I purchased the camera when I register my Nikon equipment with Nikon Australia.

For me, photography is a means of telling a story using picture(s). How those pictures (photos) are derived or processed is inconsequential. It is the end result that tells the tale and leaves an impression on the beholder IMHO.

When it is all said and done, ask yourself why you selected the mobile phone you are using now. Was it primarily because it takes great photos/videos or its ecosystem and the camera capabilities came along for the ride? We can thank the mobile phone manufacturers for putting such amazing photographic capabilities into smart phones because it has introduced millions of new photographers into this field that otherwise may not have been interested in photography had such capabilities been omitted. It also makes taking images and the comparative effortless ability to transfer those images elsewhere a straight forward process which some of us need in running our busy lives.
The major side effect has been seen in the demise of the dedicated point and shoot camera which was a good income earner for the camera manufacturers.

However amazing as mobile phone cameras are, they are not designed to be held for easily taking photographs as is a dedicated digital camera or have the focal capabilities of such cameras though they often come close in that regards.

Only time will tell whether the traditional dedicated camera will survive. My hope is that a small percentage of folk now discovering the joys of photography had it not been for the acquisition of their smart phones will move on to owning dedicated photographic gear that a lot of us here UHH enjoy using.

There is another way to capture a wide landscape in more detail and with a more natural look without the shrinking effect wide angle lenses impose on the background of the scene and that is to stitch together two or more photos to give a panoramic effect. This is what we unconsciously see when beholding the breadth of such landscapes and wish to somehow get all that lovely beauty into a standard sized shot. Can't be done.

A feature some forget is inherent in their D850 and similar camera bodies is the ability to extend the apparent focal length of the mounted lens by x1.5 using the menu system to select DX crop over the default full-frame FX at the expense of having a photo with less pixels. Sometimes this can "save the day".

Below is a panorama of a small section of Sydney harbour taken on a windy day looking back at Camp Cove Beach near South Head, the southern headland, as one enters Sydney Harbour from the Pacific Ocean. There is no way such an expanse could be captured in one standard sized shot.