Don't get me wrong, ZA is a great product, it works well and they made a free copy too. Can't really fault them for that. I just wish more people used it

In the past I had issues with it , could be that those are now fixed.
With multiple PC's behind the router, quite often the router acts as the DCHP host and assigns IP addresses for example starting 192.168.1.1 to 256 to whichever PC comes online first.
You'll need to assgn a static IP address to both PCs then have the router point port 21000 to the IP address of the host PC
Next question is how to assign the IP address to the PC, but it's Sunday evening, I've had a shit week, I wanna fly .....
G