MTN ***** money

On the 13th September 2019 I went to MTN to take out a "Mega deal" 120gig for R399 PM X24. They said to me they don't have any modems in stock I must come back the next day, I went to MTN River Walk Three Rivers the next day to collect my modem but they had hundreds stories and I went home without the modem. On the 3rd Ockober 2019 I finally received my Modem ( 20 days later). My first Bill come and they charge me R1018.73. I went back to MTN Three Rivers to query it but AGAIN there systems were offline and they couldn't access my account, while I was in the the store another guy come in and they told me they are struggling for a year now where MTN charge them double every month . They have logged a ticket for me with the billing department but still haven't received any feedback why they charged me R619.73 more.