RealVNC on Remote Network

I can connect to one remote PC using RealVNC with no problem. But
when I launch the VNC Viewer (client) on the remote PC and enter a
valid IP of one of the other PCs on that remote network (which has the
RealVNC server software), it draws its screen but then stops: I cannot
control the second PC. Any ideas? Do I need to change the RealVNC
port numbers for each separate remote PC on that network? I’ve tried
Radmin, NetSupport Manager…still looking for the best solution of
remote admin! Thanks, Barry