If you are using the free version of NoMachine or Enterprise Desktop, you are connected to the physical desktop of the remote computer.
In this case the maximum resolution you can get is the maximum resolution supported by the remote graphics card.
Linux remote hosts
If you need to work at a higher resolution and your remote host is Linux, please consider using one of the products which supports the virtual desktop functionality, e.g. NoMachine Workstation. The supported resolution for virtual desktop sessions doesn't depend on the remote hardware and the remote screen can be resized to whichever resolution supported by the user's monitor.
Working at a higher resolution is also possible on a headless machine. In that case, turn off your X server and let NoMachine create the virtual display for you.
Instruction how to disable Xserver you can find there: https://www.nomachine.com/AR03P00973
Further reading: Resize and scaling options for NoMachine client version 4 or later https://www.nomachine.com/AR02M00835.