Tesserax appears to be offline today, so I will answer.
He did provide you with the detailed steps to reset your Time Capsule and set it up again....and we asked where in the process that you were getting stuck....to try to fix things. We could not seem to get a clear response from you to work through this snag.
There are different ways to skin a cat, so if you want to try another approach that basically does the same thing, the steps are listed and illustrated below. I have added a few details as well.
Make sure that an Ethernet cable connects from one off the LAN <--> Ethernet ports on the ATT modem/router (there are probably four of them....choose any one that you wish) to the WAN "O" port on the Time Capsule (TC).
Power up the TC, if it is not already on, and leave it on for a few minutes
Hold in the reset button on the back of the TC for 7-8 seconds. The amber light on the TC will blink quickly during this time. Release the reset button after the hold period and allow a full minute for the TC to restart to a slow, blinking amber light status
Click on the WiFi menu at the top of your Mac's screen and look for a listing of New AirPort Base Station. Just below that, click on Time Capsule, or it might read AirPort Time Capsule.
When you click on AirPort Time Capsule, that will start up the AirPort Utility setup "wizard". It will take a few seconds to analyze the network and then display a screen that looks similar to the illustration below, except that you will see a Time Capsule pictured.
Network Name......Type in the same wireless network name that you used before
Base Station Name....Type in a device name for the Time Capsule
Password.....Type in the same wireless password that your network used before
Verify....Type in the same password again to confirm
The setup wizard will set things up for you automatically. When you see the message of Setup Complete, click Done.
Now click on the WiFi menu again at the top of your Mac's screen and then select the name of the Time Capsule, and enter the password if required.
If you want to use the Time Capsule wireless network, simply do not connect to the ATT WiFi network. If you don't plan to use the ATT wireless at all, ATT Support can tell you how to turn off the broadcast of the ATT network, so the only WiFi network that you will see will be from the Time Capsule.