For SMC7904WBRA :: How to use Cable Modem on ADSL router/modem
Looks like your device does not support playing router for cable internet. Would recommend you get an actual router, or alternately if you have a computer with two network adaptors you could plug one into cable modem, plug other in to LAN side of router, then configure turn Windows Internet Connection service, will need to disable the DHCP service on the router. I highly recommend you get a real cable/dsl router to do this tho. Linksys 54g series has been one of my favorites.
Router homepage on SMC site.