Look for the factory reset button on your AT&T U-Verse device. The reset button is usually red in color and located on the back of the device.
To initiate the factory reset, press and hold the reset button for at least 20 seconds. When you release the button, the device will begin the factory reset.
Allow some time for the reset to complete. After about 5 minutes, restart the AT&T U-Verse device. Your router should boot up with factory default settings.