From few weeks my s21 ultra start to disconnect from my network. I was with it at samsung service and they said the problem it's fixed. I using voxi sim and they said it's from voxi signal and today I used sim from EE, same problem.
I have the same thing on my s21 ultra. Changing to 3g/2g auto is slightly more stable, but can get days when it's terrible in all states. I've taken to just switching to airplane mode and switching WiFi and Bluwtooth back on, just because it constantly connecting and disconnecting from the mobile network drains the battery massively. It's been factory reset, and also been back to Samsung and apparently 'fixed' but it did nothing to fix the actual problem. Changed SIM too, and my usual SIM works perfectly in other phones.