I would like to have the same subnet at the end of my VPN. I have two locations both with UDM PROs set up as a point-to-point VPN. Anyway they both could run off of the same DHCP server so both networks bridged with a PTP VPN could all look like they live together in one subnet? If not how do I get each subnet to understand who is on the other subnet? I can ping things by IP address but no host names. I am not able to adopted new APs in the other network and I would like to be able to.
OpenVPN does have a mode where it operates on layer 2 instead of the usual layer 3 mode. I don’t know whether UDMs support that, though. pfSense does support it, but I have never deployed it.
If a host want’s to send an IP packet to another host in the same network, it resolves that host’s MAC address using ARP and sends the Ethernet frames directly to the destination. Only if the destination is in another network, the Ethernet frames are sent to the gateway. So my guess is: you don’t. At lest not easily.
That should be a matter of configuring DNS correctly. You would typically have one domain (DNS suffix) per site. Each site can have its own DNS server responsible for the names that live on it if you want. Then you can instruct the (recursive) DNS servers to use the other site’s DNS resolver for its domain, respectively. In pfSense that is done in the “Domain Overrides” section of the DNS resolver, I don’t know if the UDM has something like that.
Are you aware that you can adopt most Unifi devices manually by connecting via SSH and providing the IP address or domain name of the controller? You can also resolve the hostname unifi to the IP address of the controller.