Welcome to the Community @Tharakabandara
In order for you to use your Google WiFi, you need to set it up as a WiFi extender / Access point. Set your router (Google WiFi) on a Dynamic/DHCP connection. Refer to this link for more information.
Connect your Google WiFi to any of the LAN ports of the TPG provided modem/router, then check if it's going to work.
BasilDV
