Apple won’t replace 4 month old iPhone X

I have a 4 month old iPhone X. It’s faulty, it works intermittently, it’s works on 3g and WiFi but not signal bars. There is prepaid sim in the phone, I can make calls intermittently. I have taken it to Apple and their response is the phone has been blacklisted. NO the phone has not been blacklisted. I have checked with my service provider 4 times. They have confirmed that it has not. I want this is phone replaced. Why do I pay r22000 for a phone I can’t use and Apple won’t fix it. Apple are not coming to the party to sort this phone out. The handset is faulty.