NEW PHONE IS BROKEN

I have a big problem with MTN ,

My contract upgraded on the 13/07/2018 I then upgraded and received a phone a week or two later the phone was faulty it kept freezing and I could not use the phone the screen then ended up having some black spots and when I went into the store for repairs I was told I cannot be assisted as I need the invoice that I signed in order for them to help me . The upgrade was done online and the only document I signed was when the phone was delivered which im assuming was the invoice but no one told me that I need to hold onto that paper for 2 years . I now cannot use the phone neither can I get it repaired so I don’t know what do .

I cannot understand how my contract details or information cannot be used with that invoice as I have the IME number on the phone box .