That isn't a node-red question of course. You could use the ip route command to tell the system to route comms with the PLC ip address through eth0 (or whatever is the ethernet device). I don't use that command often enough to know the correct syntax so I would have to google how to do it.
It is much easier if you can connect the PLC to the network rather than the Pi, but I realise that may not be practical.
I was able to guess that might be the problem because I have spent the last 50 years working in IT. You can acquire such knowledge by doing the same, or at least making a start down that path.
