non functioning phone

to whom it may concern, my name is Patricia i bought a Samsung 03 core phone on the 24-12-2021 at pep store in centurion, where i was told that i should not return it for any reason now the phone is giving me problems, it switches off anytime and now it doesn't charge properly and it doesn't go on at all therefore i went Vodacom shop to have it checked and i was told that the phone was a demo which i believe because at the time of purchase i was told that there were only two phones available on that day then i asked the cashier that assisted me to keep one for as i was rushing to the car to get the proof of residence and there was one customer who also wanted the same phone and on top of that i had to drive again back to the store to get the pin that opens the phone as it was not in the box, now I'm sitting with a phone that is not working. kindly advise or assist.