This indicates your "Modem" is setup as a network gateway, i.e. your routes are pointing to it as its default gateway. If you have your own router, and still have the 2wire setup, you can have ATT set the gateway interface to router mode, where it will only pass traffic to the next hop. I.e. it will hand the public IP off to your personal router. A work-around for this issue is to setup your own router, place it in the DMZ. Assign the ip of the router 1 numeric value higher than the gateway. (eg 192.168.1.1 is the 2wire, set your WRT to 192.168.1.2 with a default gateway of 192.168.1.1). You'll need to be sure to shut off the firewall on either the 2wire or on the WRT, you shouldn't run both SPI firewalls simultaneously. Also, dhcp on the 2Wire will need to be turned off, and DHCP on the WRT enabled. You'll need to add DHCP exclusions (I generally exclude 192.168.1.1 through .25, reserving the range for static IP devices). The most important step here, is to ensure the DHCP scope has 192.168.1.2 as its default gateway, otherwise it'll talk directly to the 2wire device. At this point,you have two choices, you can set a dhcp reservation for the apex, so that it has the same internal IP address all the time, without having to manually configure the IP. Or you can leave it in dynamic mode. Since you'll be using the WRT for a gateway, it will handle its own local dns registration and resolution. DynDNS can be used in the WRT at this point, as now it has the public IP.
I spent *days* screwing around with using the RG in the aforementioned method, and I went into a deep/dark depression. Maybe something there was something screwy with my hardware/setup (or me mentally, which is most likely) but I gave up and just use my RG to serve as the wireless AP. You go through a HUGE workload just to be able to offload your DDNS assignments to your router. It's not worth it unless you need more from your router than i did (e.g. 802.11n or any of the other services DD-WRT provides).