If you can’t figure it out, you can always use your PiHole as a DHCP server.

If I go into WAN settings, there are fields for DNS1 & 2 but if I was under the impression that these should be set for upstream DNS.

Try set those to your PiHole IP. Then, even if a DNS request goes to your router, it should send the request to PiHole rather than the ISP’s DNS servers.

By the way, I’d recommend running two PiHole instances so that the internet doesn’t break if you have to take one of them down. There’s a system for AdGuard Home that lets you keep the config for multiple instances in sync - maybe there’s something like that for PiHole too.