Unsatisfactory service from MTN midlands mall

I have been a customer at MTN since 2016. Last year in May 2019 I renewed my contract at MTN midlands mall in Pietermaritzburg and took Huawei P30 lite which I used for a few weeks and it started giving me problems. I returned it to the store, I was given a new one. After a few weeks of using it, the same problem I encountered with the first one started again. I returned the phone again they took it, still to this day they haven't returned the phone. I have been there for quite a number of times to enquire about it but I'm still not getting any help, yet I'm expected to pay every month for a phone that I don't even have. It's been over 3 months. I have spoken to every consultant there, even both of the managers they're all failing to help.