I had failed to put an IP address on both sides of the VPN.
Resolved by going into “VPN” > “LAN-to-LAN (OpenVPN)” > clicked on my VPN entry then in that section – there are buttons to the right (use the sub-scroll bar to scroll down to “Add IP”.
The VPN IP address is internal and not accessible/usable outside of the VPN.
It appears to be only used for route table entries and can be anything you like. As long as both ends are in the same network.
I used 192.168.123.1 on end A and 192.168.123.2 on the other end.
Once both ends had IP addresses, I was able to happily add the route.
Say my Site B network is 10.33.123.0/24 and Site A network is 10.33.124.0/24…
On Site A’s ZeroShell, I add the route:
I choose “gateway” (not Interface): 192.168.123.2, metric 0, interface=can’t change this.
Then on Site B’s ZeroShell I add the route:
I choose “gateway” (not Interface): 192.168.123.1, metric 0, interface=can’t change this.
Then it works. If pings between servers on each end fail – check your host firewalls on said servers. My pings were failing – was due to the lovely Windows firewall being enabled on one side.
Hopefully this points you in the right direction and gets you through.