Ok sorry @ronski misread earlier post. And also confused John sorry.
Therefore original suggestion will work but the second users IP ( or range) will have to be in a rule as effectively the port is closed except to specified IP’s. Sure that’s this is possible with your router, but if not I have a Draytec 2820 old but excellent I could offer on sale or return, via swap shop, but you would need a modem or bridge existing one.
However as said in earlier the rules worked really well for me, no intrusions at all. Unfortunately the common ports scanned are 25 and 3800 up to 5000. It’s a pain, but solvable.
Is VNC not an option at port 5900 software free from TightVnc and encrypted. Needs some configuration but very quick to do.
I’m working 2000 miles from home at the moment,and have been using my home servers via VNC.
Tony