Data disappearing

Good day This is the 4th month I'm experiencing data issues. I have a data sim contract ***) which I pay R200 every month and I get 40gigs, 20 during the day and 20 at night. From around September last year my data only lasts for about 5days and I never used to run out. When I first reported the issue at one of the MTN stores they gave me assumptions as they didn't know what to tell me. My phone needed a Software upgrade and they said my phone is running the upgrade on it's own (without me pressing the ok button) and failing as it's not connected to wifi hence my data finishes quickly. I decided to remove the sim card to another phone but the problem persisted. My monthly data gets loaded on the 12th of every month, so what I decided to do this month is to not use it at all. I've opened my other phone to check the balance and within 48hours it's already 10gigs left. From that sim I have a data share of 5gigs to my kids which stay in a different province. This is a huge issue as I've come across a lot of people experiencing the same problem. Funny enough if my payment is late by 2days I get messages threatening to terminate my account. I need this sorted out immediately please.