I’m on First Net (AT&T) and have the same problem with iPhone XS Max. I’ve found a workaround is to open the Tesla app and while it’s trying to connect switch the phone into airplane mode long enough for the cellular radios and Bluetooth to turn off (around 5 sec).
Then turn airplane mode off and the app will connect to the car (MS) when service is restored.
Then turn airplane mode off and the app will connect to the car (MS) when service is restored.