From settings go to general maintenance > Reset > Reset Network Settings
BEAWARE THIS WILL UN INSTALL YOUR WIFI AND BLUETOOTH DEVICES AND AFTERWARDS YOU WILL NEED TO RE INSTALL YOUR WIFI AND BLUETOOTH DEVICES.
I had the same problem & this solved the problem
But if problem still their, I would replace your SIM card, & make sure Tmobile reboots it, before you leave and try it
Thank you, Patriot1776JG. Resetting network settings doesn't work for me. I switched to another SIM card as well. No luck, either. Now my phone loses the very basic functionality! :(
Found out the solution by myself. It was the theme which caused all the mess. I was using a theme called SundayMorning. After the Android 11 update the phone and message app crashed with this theme. I switched to the default theme and everything went back to normal. The weird thing is: I'm using this SundayMorning theme again and nothing crashes now.