I think that must be a Westell C90-610014-03 after some trivial googling. Looks like it's not just a modem indeed. Westell doesn't give any information so I tried http://www.portforward.com/. See if one of the guides listed below "Westell" at the router index gets close enough.
You should probably connect to http://192.168.1.1/ or http://192.168.1.254/, and set up port forwarding there in addition to on the Linksys. If neither address leads anywhere, see if the Linksys can tell you which router it uses, and try that. You should forward UDP port 4534 as usual, but to the address of the Linksys as it appears in the Westell's subnet. See if the web interface of either device gives that information. If you can't find it forward to 192.168.1.1 if that's not the address of the Westell, or 192.168.1.2 otherwise. Of course, if the Westell doesn't use 192.168.1.x for some reason, use change the first number groups to match.
Is there anything else connected to the Westell, not including the outgoing line? If so you risk breaking the forwarding if DHCP assignments change.
