I just had a very quick play and it seemed to work after a couple of attempts - it's a while since I tested it and have forgotten the exact sequence of operations I used before. The main message I got in the car was smart key not detected, but I also had it tell me to get up and then sit in the seat again. Also the Bluetooth icon was initially blue, but then later off, even though I could use it to start the car. So that status is not of much help.
I would try the following:
Turn off wifi and mobile data so you know they're not interfering (Also useful to prove that it should still work when you're in the back of beyond), and put the keys well out of range or in a pouch.
Make sure it is 'connected' on the Digital key page, and keyless start enabled.
Unlock the car with the phone via the Digital key page
Press keyless start on the Digital key page
Sit in the seat
Press the brake
If that doesn't work try the keyless start before unlocking or after sitting in the seat.
Also try the keyless start on the main page at various stages.