HELP with DSL and Apple Airport Extreme

I have a ATT dsl line. And it's plugged into the WAN of the Airport and ! of the Airport outs to a Vonage VOIP modem. The systems works fine, but every now and then I have to go over and reset the modem by powering it off anf then back on. When I try do resetup the Airport or add my express base station in the basement I get Double NAT errors etc???? Any help would be great.... This excact config was working fine at my old house using comcast and i never had a problem.....

Can you log directly into the IP of the DSL modem and turn off NAT and DHCP so the Airport can deliver those services?

What Modem is the DSL using? Does it say AT&T on it? What is the Model Number on the DSL Modem that plugs into the WAN on the Airport?