The NoMachine version I am using is : 6.7.6
The option for : “Full screen on all screens” is available.
When I used it, the window initially expanded only to that display.
I did try the option on the Mac as suggested : I deselected the option “Displays have separate Spaces”.
I needed to reboot the Mac. Then after a couple of tries, I was able to get the “Full screen on all screens” to cover across my multiple displays.
Thank you very much for the pointers.