the tutorial, or Welcome screen, is a generic one showing all the icons that are available across all the supported OSes. When multi-monitors are attached to the computer you are connecting from there will be two icons specifically for handling how to view the NoMachine window in relation to the monitors attached. The first one with numbers (so you can switch the view) and position 6 which makes the NoMachine window go full-screen across all monitors.
On macOS it’s a bit different mainly because Spaces does not allow for windows across more monitors. You could try disabling “Displays have separate spaces” on the Mac machine as a workaround.
You wrote you have “two screens on the server side and two screens on the client side”. To be able to treat multiple monitors connected on the server as separate windows on client side is actually something we are planning to do. This will allow the user which has connected and set up multiple monitors on the server side to display each remote monitor in a separate window and place that window in any desired location, including on the monitor of your choice. As yet, we can’t provide an ETA.