Hi,
with NoMachine you should not have to be already logged in in order to get access. This defeats the whole point of using it, as you say.
There is a known bug when using an AD domain account to log in for the first time with NoMachine. This could be what you are experiencing – https://knowledgebase.nomachine.com/TR04R09659.
If this does not apply to you, ie. you are not using an AD domain account, tell us what the message is when you try to log in with NoMachine (a screenshot will suffice). Logs from the player (client side) and the server (computer you are connecting to) might be useful. Follow the instructions here:
https://knowledgebase.nomachine.com/AR10K00697
You can send them directly to forum[at]nomachine[dot]com.