iPhone 11 network problems it always says unknown error when I try to buy airtime with ussd

I received my iphone11 I bought from the Appleguyza on 12th November while I placed my order on 1st of November. Upon receiving it I realised that I’ve been missing calls even though my phone is on and I’ve never had a problem with network where I stay.So I tried reaching out to them because it’s not even a month since I received the phone but they not answering my calls and also ignoring my Facebook messages.i am a frustrated unhappy customer.