When you connect from an external network or over the Internet, you need to use in the connection settings the external IP and the external port of that server host.

By default the automatic port mapping on the router is disabled in NoMachine, also in the free version.

To enable it, go on the remote computer and do the following.

Open the NoMachine interface -> Settings -> Server -> Ports and select the nxd service, then click on Configure:

In the next panel, check option 'Use UPnP to map the port on the gateway IP':

This should configure automatically your router for the external IP and the external port.  Use them in settings of client connections.

If the automatic port mapping fails for some reasons, you will have to configure the router manually. Please refer to:
https://kb.nomachine.com/AR04S01122