How do I open/forward a port on an Arris TG1672G?

Per the results of a traceroute, I can see that the device I want to forward the port for is behind a Netgear WNDR4500 router which is, in turn, behind the Arris TG1672G modem. Per step 3 at https://openmyip.com/Arris-TG1672G-router-setup, I first need to assign a static IP address. Using the same IP address that the Netgear router knows, however, I get an error message that it is "an invalid IP address."

The device's address from the router's perspective is 192.168.1.5. I am confused as to the IP address assignment process from the modem's perspective. The modem's address is 192.168.0.1 whereas the router's is 192.168.1.1.

Solution 1:

Thanks to @Ramhound's feedback I was able to figure it out. In a nutshell, I needed to view the router as a device on the modem's network. I was incorrectly thinking of it all as a single network and not grokking the concept about "subnets." So, opening the port on the modem for its static IP address for the router made it all hum.