Unfortunately, Apple assumes that you will be using a simple modem with their routers. Something like this, for example, which will allow the Guest Network feature to work correctly, since the AirPort will be the main router on the network.
The Uverse is a modem/router, also known as a gateway......a device that combines the functions of a separate modem and separate router in one package. An example of a modem/router would look like this:
Because the Uverse is already operating as the main router for the network, the AirPort cannot take that role. So, the Guest Network will not function correctly with the type of device that you have.....even if you try to enable it in AirPort Utility.
Suggest that you use the AirPort router as your main network, and use the Uverse wireless for "guests".
Sometimes, it is possible to reconfigure a modem/router to act as if it were a simple modem. I do not think that this is possible with a Uverse product, but you could check with ATT on that possibility if you wanted to do so.