The latest version of the iOS 9 beta has brought some goodies, among them the new Wi-Fi Assist feature, which will recognize bad Wi-Fi connections, and use cellular data instead.
https://gizmodo.com/ios-9-preview-well-this-was-long-overdue-1715248706
You can switch it on and off with a toggle in Settings, so if you’ve got spotty home Wi-Fi — or jealously guard your data like your do your first-born — you can disable it. Otherwise, it will just detect when you’re using a network with limited or no connectivity, and use your cellular data instead. Before it switches, the Wi-Fi signal gets greyed out, giving you (some) notice that you’re about to start using data.
Other new features include a new range of wallpapers, and the option to use Wi-Fi calling on AT&T (although AT&T hasn’t turned the feature on yet, so at the moment it’s just a pointless button).
