Apple has added a new function WiFi Assist, for some people complain that when the WiFi signal is poor, they will use more cellular data without awareness. "Because you'll stay connected to the Internet over cellular when you have a poor Wi-Fi connection, you might use more cellular data," Apple cautions in one part of the document. "For most users, this should only be a small percentage higher than previous usage."
Wi-Fi Assist is turned on by default, and is meant to ensure a smooth Internet experience. Because it switches over to cellular whenever it decides that a Wi-Fi signal is weak and not simply absent, however, this can potentially cause people to exceed carrier data caps without realizing, according to AppleInsider
The document notes that certain limitations have been applied to the feature. It won't switch on while data roaming, for example, and only applies to foreground apps rather than background downloads.
Likewise, not all apps can take advantage. It shuts off for some unidentified third-party apps that stream audio or video, as well as some email apps, since they might download large attachments. Apple's own apps typically do make use of the technology, including Mail, Maps, Safari, Apple Music, and others.
To shut off Wi-Fi Assist, iOS 9 users must go into the Cellular menu in the Settings app, and scroll all the way to the bottom to find the toggle button. The option is missing on some older Apple devices, including the iPhone 4S, the iPad 2, and the first-generation iPad mini.
This new function is helpful to solve your cellular data excessive use.