So my iPhone wireguard ip is 10.13.13.2. My ‘DNS server’ in the Wiregurd app is set to 10.13.13.1 (Wireguard server on my RPi). I have also made the ‘DNS server’ option in the app my RPi IP address as it uses Pihole/dnsmasq to resolve.
I have set Pihole to listen on all interfaces just incase that was causing and issues.