To disable Wi-Fi Assist in iOS 9, head to Settings > Mobile Data.
The contents of this menu will depend on your carrier and the number of apps installed on your iPhone, but scroll all the way down to the bottom of the page and you’ll see an entry for Wi-Fi Assist. Tap the toggle switch to set it to Off (white).
That’s it; there’s no need to restart your iPhone or change any other setting. Your device will simply behave as it did prior to the iOS 9 upgrade and will only switch to a cellular data connection when it completely loses its Wi-Fi signal.
If you ever want to turn Wi-Fi Assist back on, just return to Settings > Cellular and toggle the switch back to its default On (green) position.