iPhone 12 SMS messaging problem

I have the same issue on iPhone 12 on iOS 14.2, on AT&T. Not receiving messages in group texts or one-on-one SMS. Spent 2 weeks with Apple Support and still having this problem. Really bad look, there's no consistency to who I'm not getting messages from either.

I’m running iOS 14.2, iPhone 12 Pro on Verizon and I’m not getting replies to group messages. As above, no rhyme or reason regarding which ones I get and which I don’t. I may not be getting individual messages as well, but you don’t know what you don’t know.

SAME! So irritating! Asking for new phone after 2 weeks of no fix and just getting run-around!