Hi Svens,
SSH is not supported in the free version of NoMachine, so if you want to connect using NoMachine (free) over SSH you should look at the article here: https://www.nomachine.com/AR10K00728. Otherwise you will need to use the Workstation.
If you want to do some tinkering with the NoMachine server software, I recommend you take a look at the configuration guides aimed at the Enterprise products (Workstation, Terminal Server and so on) for example, https://www.nomachine.com/DT09K00059. All the documents are listed here: https://www.nomachine.com/documents.