I can confirm that YoloRepair's fix does work. I did it on my phone today that had the greyed out Wifi issue. I want to add and clarify a few things that might help the next person:
Watch this video before doing anything: [http://youtu.be/ZHH-zbNgJx0|http://youtu.be/ZHH-zbNgJx0] You don't need to follow all the steps, just the ones on how to open the phone, disconnect the battery, and remove the EMI Shield. That's all you need to do to disassemble. You will need a pentalobe screwdriver to open the phone and a spudger definitely helps with prying and whatnot. You can buy them both of course on iFixit.
I disassembled and using a hot air station, put hot air on the wifi chip and heated the surface to almost 400 deg F. I used a heat sensor to read the temperature as I was heating it up. It only took maybe 5 seconds. I would strongly recommend this method over a heat gun as the air temp is easier to control and put down on the chip. You don't want to heat anywhere else, and you don't want to overheat and potentially blow the chip or other components off the board. That is it; i reassembled and wifi turned on and has stayed on and has stayed connected thus far.