Dual Phone Lost Signal

I bought a Samsung A02s 2 Months ago at Markham. It is still under warranty and it is a Dual Sim phone. I was advised by the Sales Lady that it will still function properly even when the main Sim is not being used. So, we have put Telkom Sim in the 2nd Slot. And Main Slot belonged to MTN which was not in use. 2 Months later the main Sim lost signal and the message that was displayed on my phone was the IMEI number and some " Your sim card can't be detected or it is from a provider that is not supported. I then went to MTN in Polokwane near Game for help. The person who assisted me said " You are not the first person to come with such a problem rather you take it to some place and they will fix it for you. I said but isn't it MTN who is supposed to solve the matter? She said No. I went to the Shop that I was referred to. But they charged me some ridiculous price and I went back to the MTN shop and the Lady said ok. Let me help you fix it. An hour later she showed some signs of winning but just when she was done doing Factory Reset. The phone went back to the same screen. It was logged and same message was displayed. I want to know from you Petet that ...Who is supposed to fix my phone. Is it Markham or MTN?

Please Help.