Mtn multiple debit orders

I took out a home internet month to month contract with mtn. I had to pay R1251 upfront before mtn would deliver the router. The router was delivered on the 11 June 2024. On the 13 June 2024 Mtn debited me 6 times for the amount of R1251 for each debit. My account went into negative. Four of those 6 debits came back unpaid. My account is negative because of the balance of the 2 debit orders, unpaid fees because of the 4 unpaid debit orders and my other debit orders going unpaid. When I called they said to me to send my bank statement which I did and was told it would take 14- 20 working days to be investigated. Which is absolute bull**** because I am the one suffering I do not see what investigation needs to be conducted when mtn debited me so many times as I didn't owe them any money, I paid upfront!!!! This is affecting my entire profile with my bank and my credit score and unnecessary bank charges that mtn needs to pay. I am not going to be liable for these charges when mtn is at fault!