Data disappearing - no explanation or resolution

I have been having constant issues with my data disappearing within seconds. As soon as I purchase any data, my data disappears instantly where I’m getting multiple SMS’s showing how my data is depleting within seconds.

I went to the store to raise this, where a consultant double checked my background settings to see if anything is running or if any automated update settings are on. None of these were on. They then sent me data in store and watched my data disappear.

I now have a new phone with a new data package and once again, 6 gigs just gone within seconds !! This matter has still not been resolved. It is becoming ridiculously expensive to constantly purchase data that depletes within seconds, despite the fact that I am ALREADY paying for my data package.

Please can this matter be resolved urgently.