Unreasonable service

We bought a new Huawei Y6 2018 on the 04/07/2018 (July ) , 2 weeks after the purchase the phone started with trouble, we put the phone on charge it went dead, then suddenly the Huawei sign comes up and then went dead again, the phone did this for about 4 times, their after it was completely dead . We couldn't go back to cash converters due to personal reasons, we got a chance to go back on the 04/08/2018, and explained the problem to them, they said we didn't plugged it in the charger the right way. I told the manager we did and requested for my money back as I have a 6 month guarantee. The manager laughed at me and said we only had 7 days to bring it back. I said and showed him my invoice said 6 months, his reply was "what's written on the invoice doesn't mean anything they don't work that way. I told them they can't change how they want to if it's written on black and white and on the invoice that's the way it should be. He laughed at me and said no, we have to get it fixed by someone they won't give me my money back, the guarantee expired 7 days after date of purchase. Please let me know what can i do to get my money back or a new phone. ********** 730