Nokia Lumia 900 that is At&t locked APN settings that WORK are as follows: First from the Marketplace search and download Network Setup through Wifi.
Then enter these settings:
Name: NET10
APN: tfdata
Proxy Address: LEAVE BLANK
Proxy Port: LEAVE BLANK
Username: LEAVE BLANK
Password: LEAVE BLANK
MMS access Point name: tfdata
Proxy Address: 66.209.11.33
Proxy port: 80
Username: LEAVE BLANK
Password: LEAVE BLANK
MMSC address:
http://mmsc.cingular.comAfter this, delete the old APN and your good to go!