Incorrect Data Accounts

We are 2 pensioners each with a laptop, which are approximately 3 and 4 years old. We bought a 5 Gig data contract that includes a ZTE modem from Vodacom. (We previously had a 3 Gig contract with another supplier, which was sufficient for us). We now frequently receive text messages from Vodacom informing us that our 5 Gig data has been used up. In the last 2 months we got our 5 Gig per month, AND we twice bought 10 Gigs extra, thus a total of 30 Gigs. We have logged various service calls with promises that someone will call us back. This never happens. We only received a text message saying that we have downloaded Windows 10 which used 3 Gigs of our data. This is however not true because our laptops are too old to work with Windows 10. We definitely did not download such. Access to our data is password protected, but every now and again it is being \reset"by Vodacom"