@5spdvl
Second option requires support from your router. You have to read the manual, it can be called different names(virtual server, port forwarding etc.) Here is how it can be done:

1. You have to know your router real IP(the one that is used to connect to Internet). Lets assume it is aaa.bbb.ccc.ddd
2. You have to check your local desktop IP. Lest assume it is xxx.yyy.www.zzz
3. Rdesktop is using port 3389. If your router supports the needed feature you must configure it to forward connection attempts to its port3389 to the 3389 port of your local desktop. So if there is connection attempt on aaa.bbb.ccc.ddd port 3389, it will be forwarded to local desktop xxx.yyy.www.zzz port 3389, where remote services is listening.
4. To connect from your N900 to the local desktop you must setup Rdesktop to connect to aaa.bbb.ccc.ddd.