Paying for a device I don't own

Six months ago I upgraded my contract with MTN. The device they delivered was not what I ordered. I tried returning the device, however I was being sent back and forth by their stores and customer care line causing me to miss the deadline which I was not informed of. I did however manage to return the device with the promise of cancelling my contract so that they can start anew so that I may receive the device I requested as per MTN’s advice. They informed me that they will take care of everything since it was their fault. Unfortunately since August 2021 I have not heard anything positive about my case being resolved. To make things worse I have been paying for a device I don’t even own and nobody at MTN wants to help me. I have been calling and emailing mtn constantly since I returned the device and I’ve only been met with various excuses or no reply. I feel mistreated, especially since I’ve been trying to no avail to resolve this problem. They are purposefully ignoring me and they won’t even give me the opportunity to select the device I want.