Try installing SmartSwitch on a computer and connecting your phone. You will be able to manually check for updates in the options menu. If you don't see any, I recommend contacting your carrier for further assistance.
I switched a s20 from ATT to Tmobile a couple of years ago and had the same situation. I solved it by finding T-Mobile update file and flashing it. There is a lot of good info on XDA. It does involve using PC and Frija and Odin software and involves several steps which must be carefully followed. It's a fairly elegant solution and I only needed to do it one time. I didn't check to see if Tmobile support had a solution but that's worth a try too. Good luck
I somehow got it to work yesterday after somehow corrupting my operating system and did a factory reset. I won't share what I did since I was not successful at doing it.
