Using corrected syntax nxupnp.exe -A list 4000 -R 4000 TCP 600 I get:
Added port: 4000
Local IP: 192.168.1.3
Gateway IP: 192.168.1.1
External IP: 121.xxx.xxx.xxx
If we use this 121.xxx.xxx.xxx IP and 4000 Port combination, connection is made, ie it works. Great.
However, when starting NoMachine by clicking on the Windows Desktop icon, only an internal IP is displayed (192.168.1.3). No external IP and Port is displayed. This is the same problem referred to in my first post.