First of all, have you established whether the DNS queries are bypassing the VPN tunnel?
You can add additional configuration options in
I don’t have any experience of using OpenVPN on a phone but, generally, the (IPFire) server can “push” a number of configuration options to the client, which might include DNS (eg
push "dhcp-option DNS 126.96.36.199"). The client, in this case your phone, should then act on those instructions.
So, you need to find out if the server is pushing a DNS address and, if so, whether the smartphone is doing anything with the instruction.