The easiest way is to connect an ethernet cable from a LAN port on the ATT router, to the WAN port on the WNR2000v3 (make sure you are running the latest firmware. on mine it is 1.1.2.6
Next, on the WNR2000 V3 go to Advanced > Advanced Setup > Wireless AP
then check Enable Access Point Mode
The router will automatically configure everything else and if you want to log back into the router, log into the main router (the ATT one) and then look for what IP address it assigned to the WNR2000V3
The router will do the rest of the work for you. Make sure the router is using WPA2 for the wifi security. By default (if you reset your router), it has no wifi password set.