Firstly, dump that Google mesh rubbish in the bin. Get something decent like the TPLINK Deco M5, M9 or other TPLINK Mesh system.
This is how you need to set it up (note your modem/router does the login before the DC. You cannot have your WiFi router do the login as you have the DC between the modem/router and the Google WiFi router and bridge mode will not work)
Think about it logically, your cables come to your wall, your modem/router connects (logs you in to your ISP) you to the internet, the DC gets a connection the internet via your modem/router and then does your DPN/VPN work, your Google WiFi router then gets a connection to your modem/router and then internet via the DC.
If your Google WiFi is set to get an IP via DHCP then it is getting a LAN IP from your modem/router. If your you Google WiFi router also has DHCP enabled, then IT will hand out IP addresses your each of the devices you connect to it.
In this case, you will most likely have a double NAT and have issues. This is NOT good at all. If you follow the setup I have posted below, you will have the best setup that will work. I have also posted an image I made below. This is for a fibre to the building setup that I have here in Australia. Your WALL connection to you modem/router may be a little different however the connection remains the same. It just how it is called here.
Wall > modem/router (AT&T) > Deeper Connect > Router (Google WiFi) > Devices
Example setup,
Router 1 ( AT&T modem/router)
IP = 192.168.0.1
DCHP = OFF
WiFi = OFF
Router 2 (Google WiFi)
Connection type = Static IP
WAN IP = 192.168.0.100
LAN IP = 192.168.1.1
DHCP = ON (Start of range 192.168.1.100 - End of Range 192.168.1.254)
WiFi = ON
If port forwarding is required, on Router 1, forward the required port, E.g. port 22 for EFTP to Router 2's WAN IP (192.168.0.100)
On Router 2, for the port 22 to the IP address required of the device on your network.
Done!
