Maybe some pointers here from the official Pihole OpenVPN howto:
Since many user have asked how to setup a VPN server on their Pi-hole in order to be able to use their Pi-holes from mobile, we have extended our OpenVPN wiki considerably. It provides step-by-step instructions for many topics raised on this platform in the past:
How to set up an OpenVPN server on the same device that is running Pi-hole?
How do I set up my client devices to connect to it?
How can I ensure that only DNS requests go through the VPN tunnel, but not my regular traffic?
How to prop…
You did not mention who's doing the openVPN server bit.@James_Horn said, make sure the clients use only Pi-hole for DNS resolution and no secondary DNS configured!
Whats outcome on Pi-hole for:
sudo netstat -nltup | grep "Proto\|:53 "
Whats outcome on a Windows client PC for:
ipconfig /all
*Am only interested in the DNS and DHCP bits.
Or if on a Linux client PC:
cat /etc/resolv.conf
And on a Windows or Linux client:
nslookup pi.hole
And:
nslookup pi.hole <IP_ADDRESS_OF_PIHOLE>
And this bit:
https://discourse.pi-hole.net/t/why-should-pi-hole-be-my-only-dns-server/3376