Their generic gateway, while rather antiquated, still exceeds their advertised bandwidths.
Unless they upgraded within the last 6 months.
10/100 BASE-T Full Duplex
802.11b/g (54Mbps max)
I recommend using their gateway, then using a separate router/access point hooked up via WAN port to the gateway.
Configure the router/access point to obtain an IP automatically from ISP/DHCP, and then hook it up to the AT&T gateway.
Next login to the AT&T gateway and enable DMZ on the access point. This will ensure the gateway acts as a modem rather than another NAT firewall, allowing your router/access point to do the NATing. Disable the AT&T gateway's wireless radio, and you should be all set.
This will ensure you have no internet connectivity issues with AT&T and they will provide you with internet connectivity support, while optimizing your WLAN/LAN connectivity to your specifications. For example if they decide to upgrade your service to VDSL or Fiber, or you decide to switch to Cable, or Google Fiber comes to your town, you won't be out money on a modem you can no longer use.