Adding a route works fine - as you've seen. If you want to make the route permanent, one way to do this is add it to /etc/dhcpcd.exit-hook - per the arch-linux wiki page on dhcpcd (good resource). Another possible way to do this is to add the route to a file in /etc/network/interfaces.d. I've not tried this myself, but the contents of /etc/network/interfaces suggests that it will.
Another way to do it would be to assign your PLC an address on your LAN (I'm assuming from the wording of your question that your LAN subnet is different than the one you've assigned to your PLC). If all your devices are on the same subnet, no routing is necessary.