As mentioned above it's nothing to do with VLANs.
I successfully configured a Google Nest WiFi on an OpenReach FTTP connection nearly 18 months ago, after a neighbour struggled to set them up.
I followed advice online to factory reset the device, configure with it disconnected from other devices, then connect to the ONT.
It worked immediately and has worked since.
There's also lots of reports of these working perfectly on the BT Forums.
Google seem to just throw back any issues with "YOU HAVE A VLAN" but I can't see how it can be that.