On the many Dell laptops I've owned, the shortcuts to turn the backlight on/off is Fn+CursorRight.
Could you try that keyboard combination?
Also, the keyboard backlight can be turned off in the BIOS. This is the menu you get when you press F2 during when your laptop is cold-booting and is displaying the Dell Logo and a progress bar underneath. If it's disabled, there is no way you can use a
shortcut to turn it on, you will need to turn it on in the BIOS first.