In the Ubuntu System Settings, there is possible to enable NumLock on Plasma Startup. But when you will enable it… …it not works 🙁 It’s becouse the issue in KDE plasma.
But how to fix this stupid issue?
edit the /etc/sddm.conf file and add following lines:
[General]
InputMethod=
Numlock=on
Store the file and then make logout.
Now you can see, the NumLock is ON and problem is solved!
Good luck!