Sounds like a concept issue.
Remote Desktop needs the firewall to enable inbound port 3389 as a default. That will allow ANY pc connected to your router to use RD.
However, for some system outside your LAN (eg your work environment accessing one of the home PCs), the router must forward port 3389 explicitly to the LAN address of the target machine.
I am sure you did this long ago and just forgot about it, so log into your router and verify there is a port forward listed there.
Now the bad news is you can only PF to one machine and to see another, the router config needs to be changed.
It might be possible to enable UPnP in the router to enable an auto-config, but that would take some "inside knowledge" of how the lan was configured.