Same thing with me. I have a Nokia Lumia 928 and I thought I found APN MMS setting that worked, but then they stopped working. Then I found another setting that seems to work, but when I MMS a message to myself I get "unknown" as the sender and "message not found"??
My current MMS APN settings:
Authentication type: PAP
WAP Gateway URL: 126.96.36.199
WAP Gateway port: 80
MMSC (URL): http://mmsc.mobile.att.net/
MMSC Port: 80
Maximum MMS Size: (blank)
With that I can send, but not receive????
When I do "receive" it is UNKNOWN & "no message content"