I have a AT&T BGW210-700 router with devices plugged into the ethernet ports.
One of those is a Raspberry Pi running PiHole with DHCP. DHCP is disabled on the router.
I bought the 4 pack Google Wifi from Costco and set up a mesh network.
I can get to the internet from the wifi on the router, as well as from the Google mesh network.
I am unable to reach any wired devices connected to the router when I am connected to the Google mesh network. I can reach those devices when I'm connected to the router's wifi.
Is there something I can do to get the two networks talking?
The Google WiFi primary unit (the one connected to your existing router via Ethernet) is also a firewall & router that creates a new "inner" network inside the "outer" network created by your existing ISP router. That puts some constraints on how devices on those two networks can talk to each other. Normally, I would recommend simplifying things by having all of your wired devices connected to the same inner network as the WiFi devices by connecting them to an unmanaged Ethernet switch that is then connected to the LAN Ethernet port on the primary Google WiFi unit. At that point, that Google WiFi unit would be the only thing connected to your ISP router. This way, everything is on the same network and they can all see each other and connect without any issues.
However, in your case, that will cause problems with the PiHole, since the Google WiFi primary unit is also a DHCP server. You may be able to continue using the PiHole as your DNS server by disabling the DHCP feature and just configuring Google WiFi to use it as the DNS server instead of its default. Note that it will still be configuring clients to point to its own IP address for DNS, but it will forward all of those requests to your configured DNS server (the PiHole). This works, but doesn't give you fine-grained control on the PiHole, since it sees all requests as coming from the Google WiFi primary rather than individual devices.
In short, there's no simple solution here. But, I hope the above helps explain what you're seeing.