NoMachine client uses by default the NX protocol to connect.
Please note that installing the free version of NoMachine on the server side will only allow connections using the NX protocol.
To connect using NX protocol, follow these steps:
Step 1 - Right mouse click on the connection in the NoMachine connection UI to open the context menu.
Step 2 - Click on 'Edit connection'.
Step 3 - Choose to use protocol NX
Step 4 - Click Ok.
Step 5 - Connect to start the session.