Block access between Router and Modem

I have come across a 6 year old article here: https://www.computerworld.com/article/2887243/using-a-router-to-block-a-modem.html Basicly, for people who has seperate router/modems he suggests using the routers outbound firewall to talk to modem. Do you think it's good idea? Is there even more we should do with the firewall? By telling the router not to let us talk to it in the first place. That is, we need to configure the router to block WAN side access to 192.168.100.1. Is blocking a single IP address sufficient? I have already seen two modems that respond on multiple IP addresses. It would be safer to block any IP address that starts with 192.168.
http://dlvr.it/S263Xr