Apple has posted a support document, “About Wi-Fi Assist” which explains that with iOS 9, you can use Wi-Fi Assist to automatically switch to cellular when you have a poor Wi-Fi connection.
How Wi-Fi Assist works
With Wi-Fi Assist, you can stay connected to the Internet even if you have a poor Wi-Fi connection. For example, if you’re using Safari with a poor Wi-Fi connection and a webpage doesn’t load, Wi-Fi Assist will activate and automatically switch to cellular so that the webpage continues to load. You can use Wi-Fi Assist with most apps like Safari, Apple Music, Mail, Maps, and more.
When Wi-Fi Assist is activated, you’ll see the cellular data icon in the status bar on your device.
Because you’ll stay connected to the Internet over cellular when you have a poor Wi-Fi connection, you might use more cellular data. For most users, this should only be a small percentage higher than previous usage. If you have questions about your data usage, learn more about managing your cellular data or contact Apple Support.
You can use Wi-Fi Assist with any iOS device with iOS 9 or later, except for:
• iPhone 4s
• iPad 2 Wi-Fi+Cellular
• iPad (3rd generation) Wi-Fi+Cellular
• iPad mini (1st generation) Wi-Fi+Cellular
Turn Wi-Fi Assist on or off
Wi-Fi Assist is on by default. If you don’t want your iOS device to stay connected to the Internet when you have a poor Wi-Fi connection, you can disable Wi-Fi Assist. Go to Settings > Cellular. Then scroll down and tap Wi-Fi Assist.
Items of not about Wi-Fi Assist
• Wi-Fi Assist will not automatically switch to cellular if you’re data roaming.
• Wi-Fi Assist only works when you have apps running in the foreground and doesn’t activate with background downloading of content.
• Wi-Fi Assist doesn’t activate with some third-party apps that stream audio or video, or download attachments, like an email app, as they might use large amounts of data.
Source: Apple Inc.