you have to activate DHCP in order to make the westell a modem "dummy" (it will just act as a bridge to the internet from your Linksys.
on the modem, go to a setting that is similar to "Public Routed Subinterface". dont choose anything that says "proxy" or "NAT".
find out the Linksys's IP adress by looking at the devices or computers list on the Modem's home page.
Now, put that IP adress on the setting "Public Routed Subinterface" (or similar). If it has a "Subnet Mask" option, type in "255.255.255.0". Also, If it has a "open firewall" or "bypass firewall" option, enable it.
If it does not have a firewall option, disable the firewall on the MODEM, not the linksys. This is also called DMZPlus mode.
If the above does not work, go
here on the entire instrucitons for setting up the linksys with your modem.