I'm on the same boat as you. Using a unlock AT&T Infuse 4G with a Net10 AT&T sim. I've tried so many APN settings and still having this problem.
Here are my current settings ( I'm only showing the settings that I changed.)
MMS Proxy: mms3.tracfone.com
MMS Port: 80
Authentication Type: PAP
APN type: internet+mms