Was fighting with the same issue with my unlocked 520 Win8.1 on Straight Talk (AT&T Sim.)
Here are my settings that work...
Internet APN:
APN = tfdata
(all others blank)
MMS APN:
APN = tfdata
UserName = (blank)
Password = (blank)
Authentication type = PAP
WAP gateway = mms3(dot)tracfone(dot)com
WAP gateway port = 80
MMSC (URL) = H tea tea pea (colen)(backslash,backslash)mms-tf(dot)net(backslash)
MMSC port = 80
Maximum MMS size = (blank)
IP type = IPv4v6
Sorry about the workaround - wp thought i was posting a link and I wouldn't let me reply...
Hope it works for you!
