modem -> pc -> router [WAN]
Is this pc Windows or Linux? Windows calls it "Internet Sharing", while Linux will refer to this as "IP forwarding" or "IP masquerading".
You will also want to make sure that the pc -> router is on a different network than router -> LAN.
Example:
modem -> pc -> [10.10.10.0] -> router [WAN]
router [LAN] -> [192.168.1.0] -> internal network