Unapproved Device Claim

My device got damaged , I submitted all the relevant documents including my itemized billing now my claim was not hournered because they claim I was not using my registered sim on the registered device, which is utter lies because I was told by the MTN sales lady that I won't be able to use any sim except for the registered one now I'm surprised that they even mention that when they know very well I can't use another Sim on that device. I need assistance MTN Insurance does not want to render their services which I pay for every month.