MTN Secunda store selling fault phones

Hi I have a problem with MTN store at Secunda I brought a phone Hisense H60 and one week later the screen was flashing and not functioning correctly. Hence I went to the shop to ask the replacement of the phone but the manager told me that I am out of the seven days rule. The rule I was never told about it, during the day of the purchasing phone at the MTN store. This phone is getting worse and worse since they told me to wait until the 1st of July 2022 to get a new Hisense phone .The problem is that I do not have another phone to use while waiting. Hence I asking the store either to give a new phone with new name brand for the same price or less. The manger rejected my proposal. Please advice me on this.