Did you switch your SIM card to an unlocked AT&T phone only to find that the data no longer works?  Not being able to access the internet can be frustrating.  You may need to add or edit settings for APN, or Access Point Name, which allows you to remotely access AT&T’s network.here’s how I fixed the issue on my Windows Phone:

First, go to Settings in your phone.  Then tap on cellular.

Add apn settings (or edit them):

When I edited my settings, I only used “wap.cingular” for the APN field and left everything else blank.

You may need to adjust more fields.  More information on APN can be found on AT&T’s developer site here.