Why: Factory reset.
For me, all models have started working after that. There was this fellow that told me that pulled battery and sim card out and that helped. But so far, you've convinced me the phone has failed in some new odd way. Maybe it's time for call it a phone only?
My Samsung S4 Wi-Fi was working about five months ago. It would turn on slowly. About 2 months ago the Wi-Fi wouldn't turn on. The button would be grey until I would turn the phone on/ off. I have tried factory resetting, flashing the phone with a newer rom, turning off the Wi-Fi power saving option and so on. Is it a software or hardware problem? Maybe it is the motherboard or Wi-Fi module?