Check the AT&T modem and make sure the DHCP does not use the 192.168.1.1
I would set the DHCP to only use a range of 50 IPs like Starting 192.168.1.51 and Ending 192.168.1.100
change the Belkin back to 192.168.1.1 and turn off the DHCP. Setup you wireless security.
Now connect one of the four LAN ports (NOT the WAN/Internet port) of the Belkin to the AT&T modem