@wmh9680,
Even if it is there in the list? The list of client devices on your ATT router, or in the list of available networks on like a phone?
When the disconnect occurs, what is the status of the LED lights on the Range Extender? Does it show an active connection or wireless network? Do all devices connected to the RE disconnect or is it limited to only one device.
I would first recommend attempting to reconfigure the RE after a factory reset, just to make sure that there is not an incorrect setting causing the disconnect.
You may also try turning off the High-Speed Mode on the RE, as the bands switching due to external factors, such as interference, may be the cause.
The system logs on the RE may indicate the cause of a disconnect if it is occurring at a network level.