Disappearing data even when device is switched off

I have a data contract with MTN and for months data seems to disappear. I removed it from the router to a telephone device to monitor and this continued. When used after midnight to update it is using the normal data. In addition gigs of data are vanishing even when the device is switched off. During some months we have been receiving less data than contracted. Getting through to the contact centre is painfully impossible. Can someone from MTN please contact me to rectify