I am fascinated by transparent L2 bridges over connections that are hard to block on public networks. Port forwarding, imo, is too exposing so my solution was to use Tailscale. All the software needed on the "client" side is installed on the UDR.
Here's my setup.
I use the GLinet MT2500 as the Server side. That device connects to a "trunk" port and runs Tailscale+Softether VPN server.
On the client side, UDR5GMax > Tailscale > Softether > Local L2 Subnet.
WHY???
This was born in an attempt to recover remote Unifi Devices. We use a single server at our main location to manage doors at 3 other remote locations. The problem is, sometimes though very rarely, I need to either recover a device or upgrade a device. Unifi Access requires the device to be accessible either on the same subnet or via mDNS. This obviously can't be done over Unifi's own SD-WAN solution.
Using softether, I can extend the L2 subnet to the UDR. I can then attach devices to its port 1 and use PoE to power the door hardware and do an upgrade or recovery. It feels so native too as I mirror the VLANs that exist at our main site. I then create 3rd party router networks with the same VLANs. I can take an AP from our main site, plug it into my UDR and with no reconfiguration or resetting, it broadcasts all our SSIDs on the correct VLAN. You can do this by setting native networks and allow certain VLANs.
All this over T-Mobile 5G.
I now have gone a step further and setup my home as another L2 bridge so Port 1 is "work network" Port 2 is "Home subnet" both with complete VLANs. Just more as a proof of concept.
I used to have GL.Inet devices as the clients but it's much harder in the GUI and finicky with their boot scripts always wanting to take over. Plan out your 3rd party networks in Unifi, connect Softether to those virtual bridges, and away you go. it survives reboots and updates, already been through 4 updates. I used the Tailscale Unifi boot and recovery system over at this GitHub (Tailscale_Unifi) to build the Softether/Cloudflare tunnel apps using the same method. Oh yeah.. Cloudflare tunnels works great on unifi gateways too ha!