Data leakage causing high bills

I Dont have any more devices than I’ve had in the last year.  How could I possibly go from 900mb in a monthly average to almost 1.5TB in 4 days?  No gaming. Tried to get help, missed work to meet the tech never showed and now they want to reschedule so I can miss work again. Been with them 15ys.  So tired of everything outsourced that you can’t talk to a local person to see if this is a common issue.  Any suggestions is greatly appreciated.  At this rate it’s going to force me to take the unlimited plan ( which makes me think that’s the plan all along). Or pay $hundreds in overages.