I've got a new Mac mini - I've paired a magic keyboard with numeric field - I have tried all, but I can't use the F1/F2 to change the brightness. I use Catalina with the actual update.

I could not solve the problem by connecting with USB-cable - but I've found a software solution. "Brightness slider" in App Store.

But that it runs, at first I had to remove the hooks in presettings / keyboard/shortcuts for the keys F1 and F2.

May be it will help other - I'm satisfied with it.

Check System Preferences > Keyboard > Shortcuts > Display.

Are F1/F2 mapped to another action?

Maybe other F-keys are mapped to the brightness functions?

What display are you using? The computer has to send brightness commands to the display. For a Thunderbolt display, these go over the same Thunderbolt cable as the video. For other displays a USB connection to the display is needed.

To my knowledge the Apple keyboard brightness function works on iMacs, Apple notebooks and Apple displays only. Good luck anyways, happy to be proven wrong!

Thank you very much. I've checked that - I think the hint from Malcom, see below, will solve the problem -

I'll try it this evening - my display (Dell Ultra Sharp 4k) is connected by a HDMI cable

Greetings from Germany - and thanks for your help.

Happy to hear, glad you figured it out.