Remote Desktop doesn't work

I have the following problem: At home I have a RD connectection to a
computer (ip-adres starts with 85.203) which works. I can login on that
computer. When I try a remote desktop to a computer with a IP-adres
which starts with 192.168, I can't connect. I get the follwing message
"The client could not connect to the remote computer. Computer
connections might not be enabled....." What may be the problem?
When I am at the office I can connect to both computers. If I am at
home I can connect to the second computer through the first one. But I
want to know what the problem can be. I have a wireless speedtouch
modem. I have given my laptop a permentent IP-adress, in my
NAT-settings I have opened the ports 3389 en 3390. In windows firewall
exceptions settings I have checked the Remote desktop box. What else
could I do to make it work?

Thanks...