Hi, I was able to configure it on the live USB but once I decided to actually install Endeavour with KDE on my HDD now my DSL pppoe connection does not work. I am a newb and do not know how to troubleshoot the problem via terminal. Can you help me please?
If your termination point (modem) supports DHCP, it should just work.
In my case, it is intentional that I set up the DSL connection myself and not depend on a router.
BTW your answer does not explain why the live USB worked but the KDE install on my HDD does not.
Maybe have a look here. It might help you.
I looked at that already but i don’t really understand what to do, the DSL section is very superficial on troubleshooting and of course no sample/example commands of what to do in different scenarios.
It has been years since i used DSL. I used to install DSL systems back in the day on earlier Windows versions for many years. Dial up modem over phone line i used for years. I’m not sure what the set up looks like on live ISO that you said you were able to get working. I think i have helped someone do this once and i have totally forgotten already. I can only go by the wiki also.
https://wiki.archlinux.org/title/Direct_modem_connection#DSL_(PPPoE)
Ok I can confirm that this is some sort of bug. I installed the Xfce Offline edition on my HDD and everything works fine, my DSL connection works just fine.
Where do I report the bug?
Unless you can confirm there is a bug I’m not so sure? Xfce is a different desktop but it’s still Arch? So it could be desktop related but I’m not so sure about that. I don’t know what you had installed or set up. 
The only way would be to install KDE again and see.
Well of course there is a bug, I just confirmed it for you. Were can I talk with the actual developers?
Well i disagree with you but i don’t have the hardware to test it.
I gave up on Endeavour and Arch. After i did a system update it broke again the dsl connection on the XFCE install. Seems no one cares anymore to make sure that pppoe setup for optic fibre connections is actually working. On Mint everything works just fine. I guess those ubuntu/mint people have more time to test than arch community.
It’s not that anyone doesn’t care. I don’t have the equipment to test it. Most people are not using DSL. On Arch not everything is setup to automatically work. The ISO installs a basic set up where most things are configured to be mostly working but not everything. There are lots of hardware available and not all hardware is newer and or compatible with what is configured and may take additional steps to get it working properly. This is the nature of Arch. This is not Mint! We will help you as much as we can but we need to see logs and information as to what is happening when it happens. If you prefer to use something that is just plug and play then that is your choice. Giving up on all the advantages of an Arch based install over one disadvantage of a piece of hardware that isn’t working for you is of course also your choice. If you need any help maybe there are others here that have DSL equipment. I don’t really know.
Most people are not using DSL.
I’m using pppoe for a 500Mbps connection so it’s not the DSL you have in mind but is basically the same setup on the user’s part where you just have the username and password from the ISP. Windows can set it up since…EVER. Mint does the same. What’s the point of Network Manager in XFCE Endeavour if it does not have the same setup defaults under the hood as Mint for your typical pppoe connection?
On Arch not everything is setup to automatically work
Indeed the problem is with the mediocre troubleshooting documentation that’s… nonexistent on arch wiki.
I think the documentation is there. I can’t help as i don’t have this type of service or the hardware. Sorry, I don’t know what else you want me to tell you. I know what DSL is and how it works. Did you install the package required to set it up? Then you need to start the service. Plus all the other stuff necesary to make it work?