After much grief the following worked for me:
Be sure that your Airport Extreme is the only device connected to the 5031NV. Otherwise, both routers will try to assign private IP addresses and there will be conflicts.
This is regarding the setup where a static IP address is provided by AT&T and the ATT 5031NV is set to IP passthrough. Your AE internet settings should be the static IP address, gateway, router address, etc. The 5031NV has a LAN setting to provide a static IP with not DHCP. The firewall setting should DMZplus for your device (the only device connected to the ethernet ports on the 5031NV. Note, the 5031NV is providing no firewall so be sure to setup your own router and network appropriately.
You should be able to keep your AE on DHCP or on DHCP with NAT. You shouldn't have to use bridge mode. I turned off WiFi from the 5031NV and have created both a normal WiFi network and a guest network from my AE.