Honestly, its better to turn the router off on your ATT device and put it in bridge mode then try doing it on your d-link. But if you must, I think if you turn off the DHCP server, SPI and basically everything, and then connect the ethernet from the att device to a LAN port on the router, then use the other 3 LAN's as ports. But that will only give you 7. I don't know if its possible with different hardware revisions or firmwares, but with Revision C 3.10NA, it doesn't seem to be possible to fully bridge the WAN over to the LAN. You could load dd-wrt though, that would work, that becomes more complex.