Let's clarify some terminology which has confused you. You are using Kubuntu - by default this uses the SDDM display manager and the KDE Plasma Desktop Environment. For comparison, "regular" Ubuntu uses the GDM display manager and GNOME Desktop Environment. The display manager, as far as you need to care for right now, is the login screen that you see after boot. Before you login you have not launched a Desktop Environment. From about 2010-2017 regular Ubuntu used a custom "shell" called Unity which included a feature called Dash. You are not using this, and Ubuntu itself no longer uses this (now it just uses GNOME). Be careful when you're looking for help online - there are LOTS of Linux distributions made up of lots of desktop environments and all kinds of other stuff. Sometimes "regular" Ubuntu help applies to Kubuntu, sometimes you need to look for help pertaining to KDE Plasma (and lots of times you'll find that help from the Arch Linux wiki - Arch Linux is a very "choose your own stuff" Linux distribution so advice pertaining to KDE Plasma there may pertain to you. One main difference is the way you install stuff is different on Ubuntu (and kubuntu) vs Arch.
You've already seen and apparently followed the answer for activating numlock once the desktop environment loads. I'll recap it here (unfortunately the KDE Plasma Desktop is highly customizable and I can't guarantee mine is just like yours but I haven't changed mine considerably from the defaults - if you've already followed these directions then don't worry about it, except to maybe double check):
-click your button on the lower left (that's where it is by default - it's like the Windows start button). --click Settings --click System Settings --click Input Devices --click Keyboard
-the option to turn on NumLock on Plasma startup should be the first thing you see, but if not it's under the "Hardware" tab. Switch it to "Turn On"
Alternatively you could click the KDE "Start" button (officially it's called an Application Launcher if you're curious) and type the word Keyboard or even Numlock and it'll find the Keyboard settings option for you and you can go straight there.
That takes care of the desktop. But like we just discussed the desktop only launches after login. You want the Numlock button on for login too, which is an SDDM option. If you hunt around system settings (or type sddm into the application launcher) you'll see that the desktop environment offers you a few settings you can edit but not the Numlock one. We have to do that one manually. I found the guidance for this from https://wiki.archlinux.org/title/SDDM#Numlock (remember what I said about the Arch wiki being helpful) and https://man.archlinux.org/man/sddm.conf.5
Long story short, we are going to either edit or create a configuration file that SDDM will read when it loads to know how you'd like it configured. It appears that in Kubuntu (21.04 at least) that file doesn't exist by default. We're going to use the console - creating a 2-line text file in the console is not an "expert" task and is a reasonable expectation for a standard user here. Even if you wanted to try and do this via the file manager (Dolphin) GUI it's not allowed to create or edit these kinds of files by default and it's almost harder to get that working than to just do it. So let's just do it. I'm going into excruciating detail but this only takes about 30 seconds.
Open a terminal (lots of ways, easiest is probably ctrl+alt+t). Type cd /etc and enter (I'm not going to type "and enter" every time after this). You are now in the etc folder. This folder contains all manner of configuration files for stuff on your computer. Type ls to list all the names of stuff in the directory. They're in alphabetical order, you can go see if sddm.conf exists (by default it didn't on my kubuntu, it only would I think if you had tried to mess with the SDDM settings from the System Settings). It doesn't matter for the next step.
There are about 10 million ways to create/edit text files in a Linux terminal. People argue about their favorites - well I'm going to use a program called "nano." Type sudo nano sddm.conf - you'll be prompted for your password (by default the one you use to login). When you start a command with sudo you're running it as a "Super User" (Super User Do), this is how Ubuntu handles tasks that need admin privileges. Nano opens and either creates a blank sddm.conf if it didn't exist or opens it if it did.
I'm 99% sure the file won't exist on your computer so assuming that you now have a blank text editor in front of you. It works just like you expect. You're going to type this:
[General]
Numlock=on
If the file did happen exist, if the [General] already existed then add the Numlock=on somewhere beneath it. If it didn't, just add those two lines anywhere. Either way, press Ctrl+O to actually "write" your new text to the file (I think you have to press enter one more time to confirm the file name), then press ctrl+x to exit. That's it, you're done. Close the console or even type exit. Restart and Numlock is on
