Function F4 used to toggle between my L702x and the TV now it only adjusts brightness

I used to be able to stream videos from my laptop to the TV by hitting Function F4 and choosing to duplicate the displays from my PC to the TV but now it doesn't do that anymore. It only adjusts screen brightness. I want it to not adjust screen brightness and go back to being able to toggle between the TV and PC displays. Would anyone have a solution, please? Thank you.