I had the same issue with a 13 Pro Max on T-Mobile. Everything was provision correctly in their system, but the update to iOS 18 did something to my phone. I ended up asking them to reprovision my number and that fixed it. There was a chance that I would’ve lost my voicemails, but it did not happen. Everything seems to be working with group text with an android user in the group. See if AT&T can provision your number. it may show as having MMS like mine did at Tmobile, but for some reason, it wasn’t working. Reprovisioning wipe the slate clean and I was able to group text again.