I have no idea if these settings will work, but I came across them.
AT&T 4G Lte APN Settings for Windows Phones ( Nokia Lumia, HTC 8X ) Follow The Steps >
1-Press arrow right to access the main menu.
2-Press Settings.
3-Press mobile network.
4-Press add apn.
APN | wap.cingular |
Username | blank |
Password | blank |
*Enter pass and tap the save icon
AT&T Windows Phones MMS Settings Follow The Steps >
1-Press arrow right to access the main menu.
2-Press Settings.
3-Press mobile network.
4-Press add MMS apn.
MMS APN | wap.cingular |
Proxy address | proxy.mobile.att.net |
proxy port | 80 |
Username | blank |
Password | blank |
MMSC address | |