Very Disappointed in Ackerman's aftersales help

Very disappointed in service from akermans in , on Thursday 4 April I bought a phone mobicel XI plus . After using the phone for a few days I noticed that sometimes the battery will overheat while charging and the phone would become unresponsive I also noticed that the phone has some dead pixels on the screen which has a mark on the screen now today on the 7th of April I went in to ask them if they coul exchange the phone, after Abit of charging the manager came back and told me the phone dose not overheat , and that the screen dead pixels are just because of some apps which is not true because the dead pixels on the screen are always there even when the phone is charging and off , on the slip I am told that if I do find any fault in the device I should return the device within 7 days which I tried to do so however I was told they could not refund or exchange my device and I had to stay with that device I'm very disappointed in this service because after these 7 days pass I will definitely be out of the return offer and given more problems ,I was told more about SHAREit on the phone and the dark pixel on the screen was completely ignored the phone was not checked by a professional and only just the store manager. I'm someone who works with phones and I know that any overheating of the battery even slightly is a bad thing in the long run for the phone and the dead pixels on the screen clearly shows production factory damaged I do not know what to do as I won't be in this town for long where I bought the phone, I'm left frustrated and feeling like ive jist wasted my money If I wanted a second hand phone I would have bought a phone at MTN taxi rank. Very disappointed