I had some troubles with some newer Netopia modems and configuring them in routed subnet mode. The modem wouldn’t pass any outside traffic to our Cisco router. For initial configuration, I followed this guide:
http://www.netopia.com/support/hardware/technotes/CQG_042.html
The problem is that you have to allow the addresses to bypass the stateful inspection. This is done here:

<blockquote>Home > Security > Stateful Inspection > Exposed Addresses</blockquote>

Then you add your IP or range in the fields provided.