Poor service and repairs

I purchased a Samsung S7 edge via a contract with cell C in 2016 October, the device fell and my screen got cracked so I sent the phone to Samsung for my screen to be replaced. The screen was replaced perfectly but my device just a month or 2 after that started giving me problems. The device started to just restart without being touched and I then took it back to Samsung Sandton City for repairs having been referred by Cell C. After a few weeks my phone was repaired and was told that there was a problem with the "motherboard " of the phone. A few months after that I had the exact same problem with the phone, and sent it to Samsung Sandton City again to have it repaired and was told the same motherboard story and the phone was repaired after a few weeks. The phone has now switched off and isn't switching on and no one at cell C and Samsung knows what the problem is with the phone, but they are refusing to replace this device that seems to have a factory fault.. I have been fighting with everyone at cell C and am told that they cant do anything the problem needs to be sorted out by the supplier who is Samsung. This is very inconvenient because I am paying a fortune for the contract to be experiencing such problems.