I spent most of today wondering the same thing. I have an unlocked s21u as well but I have a sprint sim. RCS works with the default messages app. Sprint/T Mobile notified me when activating my phone last week that a T Mobile sim would be better, so yesterday I went ahead and got the tmo sim. 5g coverage was the same for me, but today I noticed the chat settings weren't there and RCS wasn't working.
Spent time on chat, in the store and on phone with them and they all said that it should work. in the end I had my sprint sim activated again and those features are enabled now. I thought the while time it might have been something with changing the sim but seeing as how you don't have it as well makes me think the s21 might just not have it as unlocked and on t mobile.
It will be interesting to hear if anyone else has same issue or if RCS is good for them on unlocked phone.