Establish a permanent Ethernet cable connection between the ATT modem/router and the AirPort Extreme. Specifically, connect from a LAN <-> port on the ATT device to the WAN "O" port on the AirPort Extreme.
You can run an Ethernet cable up to 300+ feet or 100 meters with virtually no signal loss, so you should be able to locate the AirPort Extreme exactly in the area where it will be needed.
Use AirPort Utility on your Mac to configure the AirPort Exteme to "create a wireless network" using the exact same wireless network name, same wireless security setting, and same password as the ATT wireless network.
AirPort Utility will suggest that the AirPort Extreme be configured to operate in Bridge Mode, which would be the correct setting in this case.