Faulty iPhone 6 charger

I bought an iPhone 6 at the Dion Wired Cresta branch on the 1st of March 2018. A few months down the line, the phone was giving me problems with the charger/battery. Anyway, after the phone completely refused to charge last night. I took it back to the Cresta branch this morning because it's still very early days for the phone to be giving me problems, it has not even finished a year. They refused to assist me because I used a Cell C sim card instead of a Vodacom one, I was informed that I forfeited the warranty due to that reason. Though I understood their explanation, I still would like to understand what relationship there is between the phone not charging and which sim card I use? Had the cell phone had network related problems I would understand.