Can't receive data when on 5G UC

I came for the same reason. I just switched to t mobile yesterday and right away noticed 5G UC means my connection sucks. On S21 Ultra + . I need to ask my wife about her phone but assume it's the same since it's the same type of phone we both ported over from Verizon. When it goes to regular 5G it sends "normal" but once UC shows up it's pointless to try.  I tried some of the things above and I'm on a physical sim but so far nothing concrete.  I'm going to try that airplane mode thing, years ago I had an S5 that needed that trick to work.