It turns out that I just needed to wait! I got on the phone with AT&T to troubleshoot, and it was working fine. Calling the phone stayed on LTE instead of switching to UMTS.Honestly I'm not sure, I don't use microg (nor ever have). I stick with opengapps when GAPPs are needed and I NEVER have any issues with OG, ever. I'm not sure how to help since I can't reproduce it myself (and I am on ProtonAOSP android 12 now) so I'm not on LOS anymore either.
I'm guessing something on the backend carrier-side had to propagate after moving the SIM!