I switched to an iPhone 6S and since then my data is over my limit. Long story short, I finally got a tech at verizon that explained the problem. Although I have WIFI assist disabled and I am on WIFI 98% of the time, this phone will take the high bandwidth which is Cellular, even though I have wifi in the house. You need to turn cellular data off when you are in wifi, that will force the phone to use that INSTEAD of your cellular data
sounds like your wi-fi wasn't that good & data kept you connected.
why, if using wi-fi would you disable wi-fi assist??
Because with wi-fi assist enabled, the phone will "assist" wi-fi by using cellular data.