There you go, by not working in a number of locations, you have proved the issue is in the car.
I have never had issues with the internal modem in my 2020, through what now 3 updates.... so I would take it to the dealer.
(prepare for an idiotic response the first time).
There must be a diagnostic to see if it outputs, but I doubt the dealership has it, it would need to be a cellular receiver to actually test the RF output. Stick to your guns, it took me a couple of tries for my Fiat... in my case the car could receive, but it could not transmit. I proved the app sent the honk horn command, but never got any data from the car. Again, they finally replaced the modem and bingo worked perfectly.
I had to avoid strangling people who suggested I do a complete reset on my phone.
Greg