I have an unlocked Note 20 Ultra using AT&T. It works for me. Are you using an AT&T locked phone?
Are you saying that you can't enable Chat features in the settings area of Google Messages?
https://youtu.be/NrufyCrVYkU you can also check this video out for any help activating RCS on your stock samaung message app if you don't want to use the Google messages app you can give it a try it works on unlock phones I have the s20 ultra unlock with T-mobile as my carrier and I was able to activate RCS on my phone