The Netvanta 3120 is a router with incorporated 4 port switch, not a modem. I assume you want to use it as a layer 2 switch only, which raises the question why don't you use the 'intelligent switch' in its place? In any case, the Netvanta can be configured to forward ports/protocols from WAN to LAN port(s) as required. You could even set up the 3120 to expose the 'intelligent switch' to the WAN/Internet within a DMZ.
Some links for reference: