It's a few years since I moved over to Android now, but I got my kids iPhones (using up to date iOS) so they can face time their friends etc.. On my phone (galaxy s7) there's an option under mobile data to set a data warning and another to set a data limit, I use these to ensure I don't go over my monthly allowance. I might be being a bit thick, but I can't seem to locate a similar thing on the iPhones. Is this something Apple don't have built in ?