Hello, it could possibly be your router. Some routers don’t automatically accept UPnP or NAT-PMP commands, so it’s necessary to configure it manually through its administrative interface. Please consult this article for trouble-shooting.
How to connect to NoMachine when the server is behind a NAT router or a firewall
https://www.nomachine.com/AR11L00827
If that doesn’t help, we’ll need you to submit the logs from the server side, which you can then submit here or directly to forum[at]nomachine[dot]com. Logs can be found by following the instructions here: https://www.nomachine.com/DT10O00163