Not sure if you still want to keep ATT router though. I cascaded two wireless routers. In fact the "lower" router will form its own LAN with its WAN set to be the address of "upper" router. Google for details "how to cascade two routers".
I didn't use the 2nd wireless but only wired.
I did this because I need a switch but don't want to pay for it. Someone gave me a router so I am playing around. Sure if only I want to use the 2nd router as a switch, no need for "cascade" router structrue.