Helpless David without a sling versus Goliath (MTN)

As a LOYAL customer I feel betrayed and disappointed by the service I received from MTN South Africa. I bought Samsung Galaxy Z flip 4 from MTN South Africa ( Thavhani Mall in Thohoyandou) on a 36-month contract. The phone developed issues like overheating and started with green, white and pink flickering lines on the screen after I updated it to the latest software. I took it back to the store and they agreed it was malfunctioning and they booked it for repairs as it is under warranty but then now, they are saying I should pay for the repairs and the phone is still under warranty and has no physical damage nor water damaged. I have done my research and discovered that many people experiencing the same issue with the Galaxy Z flip which clearly indicates that MTN or Samsung must take accountability for this and to this day I am still paying for the phone while they keep it with them. if anyone can help, please advice.

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