I own S20 Ultra Unlocked on Tmobile and barely just this week got the Unlocked version of Android 11 update from Tmobile, a couple weeks after they rolled out the software for the S20 Tmobile versions, but also it took longer because Tmobile suspended for a couple of weeks their release and they barely started rolling it out again
ATT barely started the Android 11 roll out for their S20 versions so it won't be long before you get it.
With all the issues people are having, are you sure you even want the update? I have a unlocked s20 and was hyped for the update. But now I think I will wait.