Cancellation of contract that is a nightmare cost blacklisting

I'm nit happy with MTN SA .There was an advert on tv showing a R499 Router unlimited data during October 2021 and My husband went to apply on the 26th of October and only got the router on the 3rd of November, It was not functioning we kept calling them and even went to the outlet at Alberton city Mall, then on the 23rd of November we decided to take it back with their approval and cancel the contract. They even gave us reference number. Since then we have called ,e-mailed, Facebook we have proof that we called and given references everytime we call in. Last week when we tried to apply for a loan we realised that we have been handed over by mtn saying we owe them.Mind you we have never missed payment on anything we owe.