Many posts note problems, possible reasons, rituals, and random solutions, but neither Google help nor this forum seem to clearly provide "how to" out-of-the-gate guidance to properly connect a Google WiFi router to the modem.
The CM1200, being a less expensive model, is just above baseline for wire-only, single IP subnet (a.k.a. flat) network attachments. Routed networks aren't flat. Blindly connect _any_ brand of routing node to the modem's Ethernet LAN and the node will likely fail to reach the Internet.
My router was previously configured and operational on another Internet service, so I knew it was working properly. A new, unconfigured router may offer some other challenges, but should be accessible once properly connected through the CM1200.
FIRST AND FOREMOST, YOUR CM1200 MUST ALREADY BE SUCCESSFULLY CONFIGURED, REGISTERED, AND TESTED WITH THE CABLE SERVICE PROVIDER.
Like it or not, you MUST do these other things to succeed at router (e.g. your Google WiFi router pod/puck) attachment:
1) CM1200 Port Aggregation feature must be in "disabled" status (factory default.)
2) Use a SEPARATE, external Gigabit Ethernet switch if you need multiple wire-attached devices because the CM1200 LAN ports 2, 3, and 4 will no longer forward traffic with the router attached. I use a NETGEAR GS305 5-port switch connected to my Google WiFi router's second LAN Port (downstream port) labeled with "<...>" icon.
3) The routing node (e.g. your Google WiFi Router) MUST be connected to the first Ethernet port. IT ONLY WORKS THERE. Luckily, the CM1200 ships with ports 2-4 blocked with plugs, so it's easy to get it right.
4) Follow reboot and connection directions in the CM1200 User Guide. With only the Internet service cable connected, restart the CM1200.
5) Once the CM1200 is restarted and operational, connect the WiFi router to LAN port #1.
6) Power up the WiFi router. WiFi access will connect to the cable Internet service because it's the only device the CM1200 sees.
7) Verify Internet access over your routed connection (wireless devices should now connect to the Internet.) If this fails, there are other problems likely outside of the CM1200.
😎 If additional switch ports are needed, they must be connected to the extra/downstream Ethernet LAN port of your router. This places the WiFi/router between the switch and CM1200, so the CM1200 still only has to deal with the single LAN device.
The procedure my be oversimplified for some cases, but it works well for Google WiFi routers. Any separate Google Mesh Points (extensions) operate wirelessly behind the router, so there's nothing to do on the CM1200 for those devices. If the Google router point works, the mesh works.
Happy routing!
