Very frustrating situation for my mom.I need solutions

My mom upgraded with MTN on June 2020 .she took a laptop that comes with router and data.she is supposed to get 12 gigs of data.8 gigs for night and 4gigs for anytime. But the sad thing is she only gets 2 gigs.while she is paying for what she is not getting. She has been trying to rectify this every month she talks talks with countless MTN consultants but even today nothing has been rectified is this not a bridge of contract from MTN. How do we fix this .