Bad customer service

I bought a new cellphone at MTN shop at The Zone in Rosebank on the 6th of November 2020. I used the phone for few hours or a day. The next day, the phone' screen was de-attaching on the top left corner & that caused the screen to crack. When i returned it to the shop, the manager told me (over the phone, without seing the damage)they won't replace it nor fix it. If they take it for repairs i should pay for the damage saying it's my fault without even being checked by a professional. Please guys help me.