I need refund from mtn at kokstad

Hi please help me.i bought iPhone 11 cash from mtn at kokstad (kzn) . Two weeks later my phone was swollen and I took it back to mtn because I had no idea what was happening. They maybe I should take it to iPhone store, I refused because it was not my fault that the phone was broken. It's been three weeks now without my phone and I don't want it I want refund because I m scared that what if something happens again on my phone? I don't want anything to do with mtn