I thought I'd give Quad9 a go.
I realised that my ZenWiFi XT 12 has the options in the Router for Quad9 for WAN DNS. So I've chosen that. I've also set opportunistic DoT and set that to use Quad9. My DHCP DNS is left to default which means it uses the ZenWiFi XT 12 for processing the requests. Yet when I go to the Quad9 test page it says I'm not using Quad9. Is this just a mistake because I haven't set it on the PC level, but instead set it at the Router level?
EDIT: After doing a dns leak test it looks like this is working fine. The servers reported back are WoodyNet, Inc. which are used by Quad9. So it is just the test page from Quad9 which is reporting incorrectly.