Dissatisfied about games service

Im so disappointed in game store in Middelburg mall in Mpumalanga. On the 6th July 2019 I bought a microwave Russell Hobbs 20L,then it started giving me problems not warming up food well or defrosting food normally, it requires you to put it at a higher level. Then on Sunday while using it, it just went off so today I decided to take it back to the shop hoping they will refund or exchange it no they tell me that I should have returned it within 7 days so now it's going to be send for repair and on their slip, it's written that you must return it within 21 days there is no where were it talks about 7days. And I don't want to repaired microwave I want a new one. Please help me, my nr is ***