No joy with Telkom over data depletion

My dad and I both have mobile wifi contracts with Telkoms. We had taken the contracts in 2013 for 10gigs of data at 10mbps download speed each. Suddenly since the contract ended in March 2015, from the month after we notice significantly reduced download speeds. I have reported this to Telkom who stated that it may be my device that needs to be replaced. Despite this we still used the system until since the past two months the data on both devices mysteriously deplete without downloading and with 10days!! My dads 10gig were depleted within 6days and with just few minutes of browsing whilst mine went from 8.3 gigs to 3.2 gigs without downloading in just 2 days. We reported this to Telkom who lodged a complaint. I then switched off my modem for 4 days and turned it on on 14/09 only to see that I now have 762mb. I contacted Telkom again for answers and they can't give me the IP addresses that took the data neither could they gives answers as to what ate the data when the DEVICE WAS TURNED OFF! Telkom has been urging me to cancel the contract hence I believe that this is a deliberate attempt to make us cancel the deal because it all too coincidental that both have data issues at once!