Frankly the wifi is the biggest and hardest area to police.
Those tablets and phones just don't always do as you expect and software doesn't always do as its told. As an example read this topic ... look closely at the screenshots of the software settings:
I would during the short term turn off the "radio" function of the router and install Glasswire on all the computers and what program and/or process is using the data.
Set up suggestions: