said by Thinkdiff:
Where/how do you use the MAC address filter? If it's only for Wireless, you can turn off the DHCP server on your router and connect it as a switch/AP to the AT&T gateway. Your MAC filter will still work without the headaches of a router behind a router.
With that said, MAC address filtering is a really poor way to do security and it's easily defeated (only takes a few seconds). With a good WPA2 passphrase, you should never have to worry about registering MAC addresses with your router.
I use MAC address filtering for wireless devices. I also use a passpharse for the devices on the MAC address filtering list. 2 levels of security I suppose