I'm with Consumer Cellular, using an AT&T network SIM card, and 5g is blazing fast here, though I'm mostly a Wi-Fi user (90% of phone time).
On top of this the S21 Ultra has superior connection compared to my two iPhones in general. I can get 5G UC indoors on my S21 where as my iPhones I can only get it if I step outside.
Another note please stop referring to having 5 bars of signal as that is just the quality of the voice connection. Try a cell signal app to see the actual bands you are connected to as well as the signal strength.
A temporary fix would be too go on 4G until the 5G situation where you are gets better. This is very common on T-Mobile Reddit.
Go to device settings. Then connecting. Then sim card manager. Deactivate your SIM and wait about 10 seconds. Reactivate the SIM.
It's helped me so I hope it helps you.
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