Almost all of us at some point have been stuck with poor Wi-Fi connectivity. Above that, the most frustrating part is when your iPhone or iPad is struggling to get hold of poor Wi-Fi. We had to go all the way to settings, disable Wi-Fi and turn on cellular data; in order to continue internet connectivity.
Apple has finally relieved all us from this annoyance by introducing Wi-Fi Assist option in iOS 9. Wi-Fi Assist is basically a tool that monitors the strength of bandwidth and connectivity; and decides accordingly whether to continue Wi-Fi or shift to cellular data. If Wi-Fi bandwidth is too slow or there is hardly any connectivity; Wi-Fi Assist will automatically move to cellular data.
The only thing to be done from your part is to turn on this smart feature called Wi-Fi Assist.
How to Enable Wi-Fi Assist in iOS 9 on iPhone and iPad
Step #1. Launch the Settings app → Locate and tap on “Cellular Settings”. (Note: On some devices the feature is under “Mobile Data”.)
Step #2. Scroll all the way to bottom of list and you will notice “Wi-Fi Assist”, simply toggle it on.
That's it, you will no longer need to go to settings, in order to disable Wi-Fi and turn on cellular data. But make sure, as this feature is a boon and a curse at the same time. If you are in middle of downloading a heavy file and Wi-Fi loses connectivity; you will not be prompted and download will continue over cellular data.