I wanna give you all a little tip, that I've found solves the issue mentioned above far better than anything Samsung recommends.
No, I do not work for Samsung, Google or any other mobile company, I'm just a basic IT professional from the UK who happened to notice this.
If you have this perpetual issue of your WiFi randomly switching itself off for absolutely no reason at all and you're sick to death of being told to reset everything (then have it make no difference at all), then I'm happy to tell you I can help. I've tried EVERY recommended fix and even had the connections board (the board inside your phone that house the WiFi, Bluetooth and USB connectors) replaced by Samsung and nothing has worked.
However, today while my WiFi was refusing to stay turned on, I tried something... as an IT professional I knew that the mobile tethering feature built into android would require my WiFi to be turned on in order for it to work.. LIGHTBULB.... the following works a treat!
1. Set your mobile tether network on your phone to never disconnect or timeout.
2. Set the connection as a hidden network, and make a complicated password to prevent anyone connecting to your device with your permission.
3. Finally, turn on mobile tethering and watch your WiFi reconnect as normal and most importantly STAY connected!
Please feel free to test and leave a comment advising which device you tested it on and what the outcome was, here is mine: