Cracked phone

Good day, I want to make a complain about Samsung. I gave them Note 4 Samsung Model SM-N910H on November 2017 for LCD(cracked screen) and the phone was working at that time. It's my daughter's phone Andiswa Mgobo and I paid them R3,800 on the 10thDecember 2017 to f ix the phone but they took time to do that and keep on calling them but there is no response, I went to Canal walk store several times they keep promising they are going to fix it, On March 2018 they said the motherboard was dead and I said I am not going to take the phone until they fix it because it is not my fault. They promise to do something about it but now they don't answering the phone even not responding to emails. Please I need your help now, I don't know what to do. Serial no:RFIFB07mgn