This thread's discussion is locked. If it doesn't give you the information you need, head to its forum board for active discussions or to start a new discussion.
Something is still using cellular data. Go into the phone settings and look for data usage and see if it will show you what was using all the data. When you are using the phone at home keep an eye on the wifi icon to see if it goes off at all.
If you find a post useful, please give the author a "Like" or mark as an accepted solution if it solves your trouble. 🙂
When connected to your Wifi, have you logged into your router to see that you are indeed on Wifi?
While on Wifi, you could toggle off mobile data to be sure a weak signal won't cause the phone to hop onto 4G.
YouTube and Internet are eating my data.
What category of use is 'Internet'? Might this be some form of tethering? Most phones categorize use by application (Youtube), but the term Internet is very generic for a category consuming a lot of data.
If you find a post useful, please give the author a "Like"