You should be able to restore your requested hot keys by depressing Fn +ESC keys simultaneously. This will toggle back and forth between the different types of functionality. You can choose the default for your function keys in the BIOS. 
So I have a Dell XPS 13 9310 2-1 and my hotkeys are not working properly anymore, I woke up this morning and I cannot turn off my brightness, adjust my volume, etc. I'm only able to open tabs and shortcuts with my F keys. 
Keyboard Hotkeys