breach of contract

I took out an contract in march 2018, 10 GB day and 10 GB night wifi router, for travel propose in April i see my data is not lasting so i went on line to see my data for the month and saw i was only top up with 5GB so i went to the shop to report, they then logged an token nothing from that in May same thing happened so i went and ask for help again nothing happened from MTN, then june came and same old story happened yet no responds yet. but the sad part is every month the debit order is going of. and yet nothing is being done. so i as a cleint feel that this is the worse service i recieved at any cellphone service yet to date. i don't want 10 GB to be happy now atleast 20GB daytime and 20GB night time for the rest of my contract due to the amount of money i spent out of my own pocket for data .