Apple fixed an iPhone bug that stopped apps from working when disconnected from Wi-Fi
Cellular data doesn't come cheap, and it's easy to run out of it, so some of us like to disable certain data-hogging apps from using cellular data, opting instead to let them only use Wi-Fi.
Except some users reported that when they go back to re-enable those offending apps to use cellular data again, the switcher will not stick to the green "on" position.
It'll look like it sticks, but when they return back to the app, they'll continue to get notices that they're not connected to a network, even though they're clearly connected to their carrier's network.
Thankfully, that problem should be fixed once you download the update.
The update also includes fixes for iMessage activation issues, iCloud backup problems, unwanted screen rotations when getting notifications, and stability improvements to broadcasts.