Faulty iPhone

Bought IPhone 13 online with mondo and the person that was helping was dishonest. She never mentioned that it’s pre-owned and the price we agreed on was R450 but they deduct R469 every month. I received this phone February and much it started giving me battery I went to MTN to report this issue, they told me to leave my phone for more than 4 hours to check the issue and I was never helped. Went to MTN again and they told me to go to store. While I was charging it battery expanded to the extended that it pushed back cover until it cracked into pieces and the screen pushed ou. When I reported to MTN they sent me to istore. I went to istore and they told me that I have to pay R1500 for back cover and for battery R 1000