I gave up on this PSU. 🙁
It simply takes too much effort and everybody's time. I will give it to a friend who I gave MB that goes with this PSU and rest of the PC to another friend, who might have/find proper ATX PSU.
During these days, I learned a lot and I would like to thank to all, especially to @momaka.

Once again, thank you all.

Have a nice weekend.

Well, glad to hear that at least you learned something new from all of this .
And sorry we couldn't get to a solution a little faster - there's only so much I can help with remotely.

momaka wrote on 2024-04-21, 16:45:

Well, glad to hear that at least you learned something new from all of this .
And sorry we couldn't get to a solution a little faster - there's only so much I can help with remotely.

No need to be sorry. I really did learned a lot. I am sorry for taking you much time.

So thx again.