I apologize for the lack of precision
I want all devices in my network without exceptions to use the VPN and AirDNS, this is where I seem to be missing something.
Your reply is very helpful, it seems the problem is that I had option domain-name-servers 1.1.1.1, 1.0.0.1 specified in my dhcpd.conf file.
I changed these to 192.168.1.1, 192.168.1.2 and now the wifi devices are also using AirDNS along with my Ubuntu machine.
Cheers!