I have a Motorola SBG6580 that is a modem and a wireless router in one. However, the wireless router part was bad so I disabled it and got a separate wireless router. I can go into the configuration pages of both the modem and the router now, and I'm confused as to which device needs to be configured for port-forwarding. I have a raspberry pi that I want to set up as a webserver.

Do I configure the router, the modem, or both?

Right now, the SBG6580's 1st LAN is connected to the wireless router's WAN, and the internet is working well. Note that the SBG6580 only has 4 ports, and I'm assuming they're LAN,as they are not labeled.