I have an industrial PC that can be connected to via a Wyse Terminal emulation application. This works fine in the local LAN.
The LAN is linked to another site via a LAN to LAN IPSEC VPN. (Each end is controlled by a ZyXel USG 100.)
From the remote site I can ping the Industrial PC, but the Wyse Terminal emulation software cannot connect. I suspect that the Wyse software is using some form of non-routable protocol, but this is beyond me. I believe that the connection is ove UDP, but I know nothing about this.