Hi! I'm having problems with my Inspire. I got the notice that ATT wasn't going to let go phone users use data without a monthly plan (the $50 or $25 per month) so I switched to one so I could keep getting data. Since then I haven't been able to access it at all.
AT&T uses at least 3 different APNs depending on how your account is setup, and the requirements of the device. If the APN on the device doesn't match what is on your account, there are going to be problems.
When you changed your data plan, the required APN on your account probaby changed. So they "old" APN entered into your device isn't going to work now. The Inspire uses a different APN in order to access the higher HSPA+ speeds (not LTE, but faster than HSPA, or 3G). You can either search around these forums or Google for the right APN for the Inspire. Or you can go into an AT&T store or call customer service, and have them fix you up.