Already found the solution... Sorry for the spam... :(

To access the F keys temporarily, you have to press the "FN" keys - this wisdom is free and most people and even I probably knew that.

But the question I had was how to change this for good so that I wouldn't have to press the "FN" key all the time. The solution was quite simple.

You just press "FN" + "ESCAPE" (with a little lock on it) and from then on you always have the F-keys for yourself first. If you press the combination again, you have the FN functions first again.

The worst thing for me was that I read online that I would have to install software to change this. This would be the absolute KO criterion for me. And no, you don't have to!
Source: https://frask.de/logitech-m...