Terrible Product Knowledge and Poor Customer Relations from Staff and Management

I purchased a Xiaomi Mi5 cell phone from the store in Menlyn. The phone was still under store guarantee within the 1 year of purchase. When phone was booked in, I was advised that if the phone is not repaired within 21 days I can get my refund. 12/03/2017 I sent my wife to collect my refund which was 21 days later. Only today do I get told that I am liable for the repair as the battery is not covered. The battery is a sealed component in the device which swelled up and could have exploded, similar to the Samsung Galaxy S4 battery issue of it swelling. Samsung accepted responsibility and repaired their devices at their expense. My issue is that this could have been resolved within the first or second week of it getting sent for repair. I have been inconvenienced and misinformed by staff at the store who do not understand their own policies. Only after I expressed my frustration did I eventually get an answer about the way forward on my device. This is pathetic customer service and I will not as a paying customer ever use Dion Wired stores again for my purchases. I would like my phone delivered to my door step with immediate effect or I will seek legal advice.