Connect the AirPort as I stated above using a permanent Ethernet cable connection.
Specifically, you would connect from a LAN <-> port on the ATT gateway to the WAN "O" port on the AirPort, and configure it to provide a wireless network using the exact same wireless network name, same wireless security, and same wireless network password as the ATT gateway.
In this type of setup, the AirPort will "extend" the coverage of the ATT gateway.
AirPort Utility, the setup application for the AirPort will suggest that you use the Bridge Mode setting on the AirPort, which would be the correct setting to use.