Occasionally I need to connect my ISP's modem directly to initiate a restart.
Normally when I connect to my modem directly, I can either connect to the modem using the vanity URL provided by the ISP, or I can figure out the modem's IP address, and connect to that (e.g. on macOS, option-click the network system menu icon, and observe the value for Router).
The problem is when I use Google Wi-Fi as my router/access point, it's no longer easy to find the IP address of the modem. Now the menu item shows me the IP address of the Google Router, and accessing it goes to a page that explains how to manage the Google Router.
How do I find the IP address of the modem which the Google Router is connected to?