MTN knows better

Found that my GPS module on my Samsung Galaxy S23 not functioning after 4 weeks of use. Returned to MTN, who had my phone for approx 7 weeks with them telling me there's nothing wrong with the phone. I was then advised by store I got it from to take it directly to Samsung for a report. Did this, after some tests they concluded that the motherboard has a fault and should replaced. After hearing my story with MTN, they provided me with a full report on the failure; stating there is a fault and a motherboard replacement is required. I then send this report to MTN together with the phone. After two weeks MTN comes back saying that the there is nothing wrong with the phone and that I must login using my Gmail account and warranty claim is rejected again. This is after Samsung and myself picking up the defect,. I've been without my new phone which I'm paying for monthly for over 7 weeks. Can someone please advise me on a way forward.