> Are you wanting all network traffic to flow through the VPN tunnel and then routed out to the internet from the VPN server or routed to the “service”?
No, just these specific (private IP addresses)
> Is the OpenVPN Server a Teltonika router or do you have control over its VPN configurations?
It's an OpenVPN server that I control
> Traffic across the VPN tunnel from one private IP subnet to anther private IP subnet does not necessarily need to have NAT applied unless they are the same subnets. You mentioned you have several routers configured with the same subnet, are they all connecting to this VPN Server?
Yes, they all connecting identically with the same IP addresses on the LAN.
> It may be that all traffic is going out to the internet before reaching the tunnel. Do you have routes configured to reach the Lan subnet the “service” is connected to?
I appears to be rejected coming from the LAN interface on the router.
> Could you provide troubleshoot file for the Teltonika router?
Sorry, I've reset it several times since.
But once I reset the router, I lost whatever other config I had set up to get it working. I think it's to do with the zones and the vpn "interface" I had set up.