Issues with Nest Wifi

I have been attempting to get a nest wifi network forwarding DNS queries to NextDNS but have come against issues where although DNS queries are working and are being logged on the website, the end devices fail to get content off most websites.

If I set the wifi to either resolve off the ISP or Automatically (Google DNS), everything works as expected. 

Is there something simple that I am missing here?  As it is just plain DNS, nothing tricky. I have confirmed in the logging on the UI that NextDNS is not blocking any requests.