I assume the Fn + Pg Up or Pg Dn key combinations only change the keyboard RGB brightness? If so, there are no controls on your keyboard. If that is the case and Windows 10 is installed, rightclick the Windows Start button and go to Mobility Center. You can change the brightness there by slider. If you have Windows 11, the quickest way is to leftclick the network/volume/battery icon. There's a slider there that lets you change brightness.