No internet connection when using pihole as only DNS server on Deco Mesh

Expected Behaviour:

I have installed a standard installation of pihole on a raspberry pi connected to my network via ethernet.
I am using a TP-LINK Deco as a mesh network.

When I set the pihole as my only DNS server in my Deco Mesh settings then I cannot access the internet and the Deco reports no internet connection.

I also get no queries appearing on the web interface.

If I offer a secondary DNS then I do get internet access.

If I set a client on my netowrk to use the pihole as a DNS server then it works as expected.

Debug Token:

OMG! I had set the wrong DNS servers in the Deco Mesh. I had set the internet connection DNS not the DHCP DNS!