We recently changed our VPN all around and are using openVPN.
It works great and no issues…except with our 1 Macintosh.
The Mac had the openVPN client on it and when connected to the VPN, the local DNS was still given priority.
Yes, the user could get to internal devices via IP addr, but not network name.
After some fiddling we discovered a few things.
On a Mac, don’t use the openVPN client, use Viscosity.
Once setup go to Preferences/Edit (bottom right)/Networking/DNS Settings
and change the DNS Settings: Mode to Full DNS
Save then connect.
That should fix any internal DNS problems you have.