You could of course return the car and order a different model if the dealer can’t program or set up the key fob so that it can close the tailgate as well. However all yourself do you love the car and wish to keep it? If the answer is yes then try the following….
While in your wheelchair can you see the button for closing the tailgate? If yes does it need a slight press or a hard press?
If you can see the button get a stick and see if you can press it.
If the answer is yes then get one of those telescopic pointer like a retractable car aerial or a slim folding stick contraption. You can use it to press the button and fold it for easy storage.
I know it is a bit like Heath Robinson but you could try this if you love the car.