There is a new feature in iOS 9 called WiFi Assist. This is a nice feature except for the fact it can eat up your data plan, which may or may not be a problem for you.
There is a problem that I have in my apartment and that is my WiFi works great except in my bathroom (I admit I look at my iPhone in the bathroom). If I don’t manually turn WiFi off and use cellular data, I get no internet access on the phone. The beauty of WiFi Assist is that it does this for me automatically.
What it does not do is tell me when it’s doing it. What this means is that you can be using cellular data, the data you pay for on your phone bill, when you think you are on WiFi. If you are someone who safely stays under the monthly cap of your data plan then this is probably not a problem. However, if you are someone who has sometimes used up all their date (guilty!), you might want to just turn WiFi Assist off and not worry so much about WiFi in the bathroom.
It’s easy to turn off. Go to Settings->Cellular-> scroll all the way to the bottom -> turn off WiFi Assist