I have one contact with a Galaxy S10 (AT&T) and until recently had no issues receiving MMS messages (photos, videos, and group messages) from me, including on my old iPhone 8. Recently, after I got a new iPhone 11 with T-Mobile, my contact can no longer receive MMS messages from me. He receives SMS from me, and I receive all of his SMS and MMS messages. He also has no issue receiving all SMS and MMS from all other users.
We've tried a few troubleshooting steps: factory reset my and his phones; checked and reset contact info on both phones, checked and reset all messaging settings; tried Textra and other messaging apps; called AT&T and T-Mobile and ran through a few steps, including resetting IMEI. No luck with anything. Really frustrating, especially for group chats.
Any ideas? Thanks.