According to several reports, it would seem that iOS 9’s WiFi Assist is a bit more sensitive than many would like. For those unfamiliar with the feature, what it does is it switches between WiFi and cellular data depending on the quality of your connection. If it detects that your WiFi is taking too long to respond or if its signal is poor, it will swap to cellular data.
This is fine and it makes perfect sense, however this might unnecessarily eat into your data cap if you’re streaming music, videos, or even downloading apps, where hundreds of megabytes could be easily consumed without you knowing. We’re not sure if WiFi Assist is meant to be this sensitive but if you’re on a low data cap, disabling it might be a good idea.
To disable WiFi Assist, all you’d need to do is head into your iOS Settings, Cellular, scroll all the way to the bottom and disable it.