I don’t know if this is an IOS 12 issue or not. I switched from t-mobile to AT&T because that is the best carrier to upgrade your phone without going into the store. Anyway, when I switch, MMS and Group messaging broke. So if I turn MMS and Group messaging off, they work, if I turn them on, they are broke. It is like the message setting went backwards with the switch. This is what I see. The morning before I switched, I was texting a group of 9 people with a mix of android and iphone. I could send pictures and sent texts and do everything normal, with MMS and group messaging on. Then I switch to AT&T and I try and sent a text to that group and it is undeliverable. In fact, I cannot send to any android group or a mix. After being on the phone with AT&T, I decided to turn of MMS and Group messaging. Now I can send texts and pictures to the 9 person group no problem. Strange.
I know, in the past, when I switch carriers, I get a “carrier settings updated” message some time during the after the switch. I have not seen that yet, and I wonder if IOS 12 beta is blocking the carrier settings updating.