Gotta love modems with built in routers. Personally, I see two ways to resolve this, one is to put the modem into bridge mode, which would have to be done by your ISP. The other is to leave the modem as a router, and simply create a DMZ for the server, which is the only device that will be behind that modem. However, I take it from your post you need to use two IP addresses. Do you have two seperate computers devices that will be behind the modem/router? Or do you need the win2k3 computer to handle both IPs?
I've searched the net looking for answers and really have not come up with any answers. I think the problem lies in the modem but i get no help or support from Qwest. They simply say they don't support the OS i am using.
This should be easy enough to work around - connect a Windows XP computer behind the modem, and get them to configure the modem such that the computer shows its IP address as your WAN IP. Once this is done, you will be able to simply swap over to the server, and copy the connection settings to the WAN-facing interface.