I had this same issue with my S5 but I don't recall what fixed it. The Wi-Fi icon at the top of my screen is constantly switching between Wi-Fi and 4G. I have had my phone for 3 weeks and I swear this just started. ATT pushed an update a few days ago. Smart switch is off. The option to auto connect to ATT hotspots is off. It's happening at home (att) and at work (Comcast). Ideas?