Pay back my money, I don't have the router!

On 29 July 2023 I got the home wifi router from MTN Northgate but because our network infrastructure was still under construction, the wifi was not connecting properly and so I took the router back to the store within 7 days as per their T& C's. Today is the 15 Oct 2024 and I still get messages to pay an account that I do not owe. I have been to the store many times to try and resolve the matter but no luck. I have called the call center and still got no help. I have dealt with Gershuine who was the store manager back in 2023, who was very rude and very unhelpful. Now Gershuine's moved to another branch in the South of JHB. When I called Gershuine he promised to assist but instead transferred me to Mpho at the Northgate who has not been of any assistance either. MTN owes me for the connection fee and sim card fee. The last I went to the branch Mpho promised to check into the matter gave me a reference number which means she's aware of the matter but never got back to me. On MTN's database of my account, the wifi account is still in existence but not in use however they still send me messages that I need to pay the account that I don't owe. I am very disappointed with the service at MTN Northgate, in particular with Gershuine and Mpho for their lack of customer service.