PiVPN / OpenVPN and DNS for clients

When my clients connect, they can’t resolve hostnames. I am using PiHole as a DNS server, and have added entries and hostnames for my network resources. Machines inside the network can use these hostnames, as my router specifies that DietPi is one of the DNS servers.

However OpenVPN clients can’t resolve these hostnames. Any idea of what change I need to make so the clients use the DietPi as a DNS server?

Maybe this will help