Are you sure you entered the correct IP address and port? In your post there is no information about which license you have. If you have the free version (eg nomachine_5.0.58_1.amd64.deb), you don’t need to set-up configuration files to enable support UPnP. If you have installed one of other products, you should enable it (EnableUPnP key in /usr/NX/etc/server.cfg file on server side).
You can check the IP by using command:
sudo /usr/NX/bin/nxserver –upnpstatus
In line starting with “NX port 4000 mapped to:” there is IP and port which you should use to connect with your Lubuntu from external network.
If you are sure that the data you entered is correct, can you send logs from server and client side to forum[at]nomachine[dot]com? Instructions about collecting logs you can find: https://www.nomachine.com/DT04M00076.