Super @Bucking_Horn for chiming in!
@clicker , you were to run below on both Pi-hole and the Mac client to see if subnet mask match:
The problem is, as you noted yourself, you lack some knowledge on important bits to be straight diving into the deep with VLAN's and such.
A method of diagnosing is to minimize.
Meaning, remove all parts from the system that are not necessary for a simple working state (resolving DNS via Pi-hole from a client).
Once you have a working state, you add components/complexity one by one (eg. involve DHCP, VLAN, VPN, own recursive DNS server etc).
Before diagnosing DNS resolution (the core of Pi-hole), you need to fix IP connectivity first (nslookup/ping/traceroute).
When have fixed the connectivity issue, you could configure one client to use Pi-hole for DNS by manually configuring network settings on the Mac client:
When the nslookup's are successful, you can deploy Pi-hole for the whole network by configuring DHCP like described in below FAQ:
And proceed by adding complexity.