What kind of phone are you using? If it is a Iphone, check your wifi-assist. Which can be found under Settings>>Cellular>>wifi-assist(at the very bottom of the screen), make sure this is turned off(Greyed out). This can cause high bills with data use. Hope this helps. If not a mod will look into this for you.
Welcome to the community!
Have you verified that your phone is using the proper APN settings? You can verify those settings here.
Have you had a chance to check your bill to see what the charges are for? You can view your online bill via My Account.
You should note that some apps and phone functions can use data in the background. That is, they can use data without your browsing. You might consider restricting background data and/or automatic updates on your phone.
Hope this helps