This is from the datasheet for your router..

The Barricade ADSL supports two modes of operation. This modem can be configured to function as bridge to support a single computer, then as the network grows, it can be reconfigured as full featured Router to provide Internet access to multiple computers.

Here's a link to the Barricade Product Page router page. Try connecting to that thing and make sure the router functions are really enabled. Sounds like from the description their not, and your just getting your ip each time from the ISP.