Anyway, when this happens to you do you notice if that little cellular connection icon in the top menubar is showing you have a strong and weak signal?. Does this problem happen when you're in a particular room in your home or is this happening in random, different locations?
You might want to try installing this OpenSignal app:
https://play.google.com/store/apps/details?id=com.staircase3.opensignal&hl=en_US&gl=US
That might help determine if want to kind of cellular coverage your phone is receiving based on where you're located.
Take a look at the APN setting in your phone and compare it to the one in the S21. to see if they match exactly or if there is some difference. If there is a discrepancy that might be due to just how each phone auto-configured its APN setting when a SIM card was inserted the first time. You might want to try creating a new APN entry matching those in the Galaxy S21, or use the APN settings from T-Mobile's site:
https://www.t-mobile.com/support/devices/not-sold-by-t-mobile/byod-t-mobile-data-and-apn-settings
(...be sure to type in everything as is, it's important not to add in your own space characters or capitalize words. And restart your phone when you change the APN so it gets loaded into memory during startup.)