Is your ATT a WiFi router?
I suggest you reset the Time Capsule and the set it up again. Same with the Airport Express. Is the Time capsule plugged into the router now? Follow Hard Reset instructions here:
Once that is reset we can set it up and then reset the Express. I recently switched the other way so I know what to do... are you there?
OK do a factory default reset then you have to choose the Airport by choosing the new name under WiFi it will be Apple?????? then you can use Airport Utility to set it up again. This is how you set it up: