T-Mo uses LTE 66, LTE 2, and NR 41 for urban speed. NR 71 and LTE 71 are long range. LTE 12 is an urban gap filler. LTE 5 is used in other areas. (NR 258, 260, 261 are urban speed bands in the mm wave region)
I have an app that shows the current band use. My Xperia 1 III likes to use LTE 66 + NR 71. When LTE 66 drops, it's dead from not being able to use NR 71 alone. It may jump to LTE 2 + NR 71, which is no more stable. It takes a long time to get onto LTE 12 or LTE 71 and it doesn't use them in aggregation with faster bands even though it should be capable.
NR 41 often causes the phone to not work at all.
US Cellular is a domestic roaming partner and they don't allow the Xperia. The provider says "Forbidden" and T-Mo confirmed that it's caused by the phone model.
My LG Velvet 5G is usually on LTE 2 + LTE 66 + NR 71 and it works fine with only NR 71.