iOS 9 is fresh out now and we all who have the right hardware are eager to know what it is all about. If you have an all in data plan you can stop reading here by now, if you have a data plan with a specific amount of data usage per month you should disable the WLAN Assist functionality you can find in „Settings“ –> „Mobile Data“. The Wi-Fi (WLAN) Assist functionality is quite useful, when you have bad Wi-Fi coverage, therefore iOS 9 decides to bring in your data connection if existent. This can bring you to the point where iOS is eating up your data plan without you knowing this, so decide on yourself if you want this function being active. It is on per default.
Image for german version: