There's a bit more to it than that.
Check to see what class C network the ATT router is using, for example 192.168.1. or 192.168.0.
Look at the addresses it is handing out. Many of the routers use .100, .101 and so on for the IP addresss on the LAN.
Hook a computer to one of the dlink's 4 LAN ports. Release/renew the network card on that computer so it gets and address from the dlink.
Log into the dlink.
Give the dlink a static LAN IP that is in the same class C network as the ATT router, but not in the range of IP's it hands out. Put that IP address on a piece of tape on the bottom of the router for future reference.
Turn off the DHCP server in the dlink.
Now hook a cable from one of the LAN ports of the ATT router to one of the LAN ports of the dlink. NEVER use the WAN port on the dlink for anything.
Now you can use the dlink as a wireless AP, and log into it at the address on that handy piece of tape, so you can use WEP or WPA or whatever floats your watercraft.