If you connect from a PC on the same network, you have to use the internal IP which you have done correctly. When you connect from a PC on an external network, you need to use the Gateway IP instead. Your office’s external IP address should be shown in the Gateway IP field (in your case 108.xxx.xxx.xx) which you obtain by opening the GUI from the menu and clicking on the penultimate icon on the right (Server settings). It takes you to the Forwarded ports panel by default. Write down this IP address along with the port number shown next to NX:…. This information is what you need to use in the connection GUI when connecting from home. Make sure that the checkbox for NX is ticked.
If you did this, and you are still getting the connection error, then it’s either a firewall issue or a router issue. In that case you will need to configure your office router to forward a suitable port to your local IP port 4000 (NoMachine uses 4000 by default).
To connect from home, you then use your office’s external IP address and the port that you either obtained from the Forwarded ports panel or which you configured manually on the router.