In your Rpi, you need to select a valid upstream server.
Putting the IP of (what i assume is) your Pi-hole as a custom, will not work as it throws it in a loop (and that's why it's not working).
Just select a few/some from the right and unless you're using unbound, disable the .12 as your custom.
If you really want to not depend of a public DNS resolver as your upstream, you can always use unbound locally and go that route.
Give this a spin:
In order for your clients to show up, you need to specify the DNS server for each client that's connecting to the network.
That can be done at DHCP server lever, or you can use Pi-hole as your DHCP server.
If however you end up using anything else (not managed by you, via a different Pi-hole instance) as your secondary DNS server then you will get some ads that will slip through.
Here's why: