It's been blanked out for me for as long as I can remember.
Couple of things.
I think ATT really has not rolled out true 5G in many places. Per their website, it's only in arenas, busy areas. So really can't limit ATT to SA mode only.
And, unfortunately, it's nothing Apple can do. Not iOS related. The Standalone option is carrier controlled. Will need to deal with ATT.
iPhone 12 models or later work with the 5G cellular networks of certain carriers. Learn how to use 5G cellular service.
support.apple.com