Generally, you just set your modem to a single IP address.
I personally use 192.168.0.3 (rather than 192.168.0.1 which is standard).
Each PC can then be assigned a different static IP address (in the 192.168.0.xx range), with the gateway & dns set to your Routers ip address.
It doesn't matter what you set your ip address as (but 192.168.x.x is recommended as these are reserved localised IP addresses).
Once you set your routers IP address & forward the ports, make sure you SAVE the settings to the eeprom, and then reboot the modem.
If you don't save it, a power failure (even for a few seconds) will cause your modem to reboot & revert back to the old IP address.