Cannot receive / send images via messaging

I purchased 2 new S9+ phones and new service.  After three days I discovered one phone would not receive SMS photos from the other phone.  Only 60% of the photos would be received and the other 40% would be received as a grey box.  This had been occurring for almost 60 days with my calling to support every week 2-3 times per week to resolve the issue.  I was told to go to a ATT store for them to troubleshoot the issue and I went from ATT store to ATT store with each one verifying the issue and stating there was nothing they could do.  One did replace the sim card but when we tested it before leaving the store the same issue occurred.   I have chatted with ATT PRO TECH with no resolution.  How do I get this issue resolved?  Does ATT not have technicians?? 

It appears there are some ATT customer service people who CARE however they have no authority to resolve verified issues.  So they apologize and or send you to another agent who will do the same thing.