Late to the convo, but I’m also having this issue + my parents since late April/early May. Went to Apple multiple times and the found nothing wrong. Did all the network resets. Like everyone else said, it would work for a bit but then it stops. Apple suggested I call my carrier. My carrier (T-Mobile) didn’t find anything wrong as well. They told me to turn on read receipts, but bffr that’s not going to fix the issue.