Bridge Mode on a Uverse Modem/Router

Hey y'all, at my business, we have Uverse coming into the main modem/router, and then have a Apple AirPort Time Capsule ethernetted off that, then about 6 other Apple AirPort extremes ethernetted off the main Apple router. All of the Apple routers are running in bridge mode, allowing the main AT&T modem/router to assign IP addresses, even though the SSID broadcast is disabled for it. This has been working fine, however we decided that we wanted to enable the guest networks on all the Apple routers. We found out that for this feature to work, the Apple Routers must be the ones assigning the ip's. My question is how do I get the AT&T modem to act as a passthrough, and let the main apple router assign the IP's. I've done some research and it appears that just "Bridge Mode" is disabled on the AT&T versions of the firmware, but did see that DMZ zones could be used. DMZ zones are completely out of my networking knowledge, and I need some help to properly set the network up so that the main AT&T router doesn't assign IP addresses anymore, but rather leaves that up to the main Apple router. Any help is appreciated. Thanks!