Samsung Galaxy S5 Back and Recent app keys topic

Hello. I have a galaxy S5 that I had to replace the display and digitizer on. Everything works now, but I realized that the recent apps and back keys don't work. They are not lit up, and do not work at all. Is there something that should be connected inside? or is there something that is possibly shut off in the settings? it's not the power saving mode, because I checked all of that. Ideas?