Not met this router, but first thought is whether the firewalls on the
computers have been set to allow inbound connections via whichever two port
you opened. As it's not the usual port it won't happen automatically.

Or, if the router cannot redirect ports to various machines, redirect all
the ports to one machine and run a portmapper on it. A portmapper is
available from http://www.analogx.com -disadvantage is that the portmapper
machine has to be left on.

Other suggestion I could make is to try VNC. Various versions are available,
and all are much easier to setup than RD, albeit slower. I use TightVNC,
though some prefer UltraVNC. VNC uses a single port in the 59xx range.

Whatever you do, look to security. Ideally, all teleworkers should use a
fixed IP address, so you can set rules to disallow foreign connections.