Welcome to TPG Community!
We were able to locate your account using your community details and learned that you are on the TPG NBN HFC technology. The bridge mode on the TP-Link modem/router will not work as the ARRIS (NTD) Box is already acting as a modulator/modem of the connection.
In order for you to use your Google WiFi, you need to set it up as a WiFi extender / router only. Just disable or turn off the WLAN or WiFi of the TP-Link modem/router and 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 port of the TP-Link modem/router, then check if it's going to work.
Let us know should you require further assistance.