Hi there,
I have an unlocked Lumia 521 8.1 DP with a Net10 AT&T SIM. Not sure if this will work for you, but here are my current settings:
Connection Name: Net10
Internet:
Access Point Name: tfdata
Network Type: IPv4
MMS APN:
MMS Access Point Name: tfdata
Proxy address: mms2.tracfone.com
Proxy port: 80
MMSC Address:
http://mms-tf.net
Network Type: IPv4
Authentication: None
I've had this setting for awhile now without any issues.