Connecting a modem to a switch port on a wireless router

I have a DSL modem and wireless router combo, so there's no actual ethernet uplink port like most routers, just a phone line for DSL with the four port ethernet switch. I bought it for when I had DSL, but now I have cable. I'd like to somehow be able to plug the cable modem into one of the ethernet ports and have the router handle that as my internet port. I'm not seeing a simple way (e.g. setting the default route to that port) to do it but I'm also many years out of my CCNA courses and don't remember what all things do what on routers as far as port redirection and such go.

The manual for the router can be found here, if it helps:
http://laureldsl.net/pdf/D-Link%2026...UserManual.pdf

I'd rather not admit defeat on this since I really don't want to have to buy a whole other wireless router when I have one right there that's actually pretty extensively configurable, definitely moreso than most consumer routers I've experienced. It seems like it should be feasible, I just lack the knowledge to do so myself and most configuration guides online are geared towards absolute newbies.