I recently updated my S21 ultra to the newest firmware version (in the US) . Since the update, my phone has been constantly dropping service or roaming when it shouldn't be. The only way to get it off roaming is to manually go into network settings and connect to t-mobile. This is a huge bug that is making me want to get rid of this otherwise good phone. Please fix this asap Samsung and DO NOT push this out worldwide until this is fixed.
I contacted support and they said factory reset it (which is the lazy persons answer when they don't know how to fix it). If any firmware update requires a factory reset in 2021, then you shouldn't be a phone company.
1) go to settings -> about phone -> reset -> reset network settings. This will clear your network and reboot it. It may restart the phone, if it doesn't manually restart it after about a min.
2) take the sim card out of the device, if you have your phone box there is a pin tool to remove the sim at the bottom of the phone. Re-insert and see if that works.
3) network mode in connections should be set to 5g/lte/3g with data roaming enabled.
4) if none of those options help, go to your local store and see what they figure out for you and maybe change your sim entirely or check your area to see if their are Tower issues.
- Jake TMO/Sprint expert