Hello @Stevep
The "primary" Google WiFi unit (the one connected via Ethernet to your existing router) is itself also a router (and a firewall) that creates a new "inner" network inside the "outer" network created by your existing router. That's why they are on different IP address ranges and why autodiscovery protocols don't work. If you want everything to be on the same network, then your wired devices will need to be connected through the LAN Ethernet port on the Google WiFi primary so they are part of the same "inner" network as the wireless devices. You may need to pick up an inexpensive (unmanaged) Ethernet switch to make more ports available if you want to connect more than one thing that way. The Google WiFi primary should then be the only thing connected to your "outer" router.
At that point, the question is whether you really still need that outer router or not. It's not the end of the world, but at this point, you will have "double NAT". If the outer router is a combination device from your internet service provider that includes a modem, then you may not be able to eliminate it from the system, but you may be able to put it in "bridge" mode so it is no longer creating an "outer" network, making the Google WiFi primary the only NAT firewall operating. This would eliminate double NAT if you are concerned about that (it's not always an issue, but some gaming applications can have trouble with it, for example).
I hope this helps you understand what's going on.
