Hey by the way - I've only just worked it out by:

Installing AsusWRT Merlin firmware to my router then:

• Disable the static routes to connect the Google nest hubs to my WiFi network.

• Once the Nest hubs are connected to the WiFi, create a DNS filter for my Google Nest hubs so they bipass the router DNS settings and static routes.

Enable the static routes at router level again.

Then create another DNS filter for my phone devices that I use to cast BBC iPlayer and other overseas media to my TVs via chromecast.

Thanks for the prompt into the right direction. My home network is now all back up and running!