If your wanting DHCP set up, you'l need to go to the LAN menu and configure your scope (range of IP's to assign).

As long as your behind a router, the only way you'll be able to get close to a real IP is if you use the DMZ port. This port will be just like you plugged into the modem directly. However I believe you'll still have a 10.x.x.x.x IP. You won't ever be able to get the IP that your modem assigns your router as long as your plugged into the router.

As long as your on the router it will always or in most cases be a private IP.