I went to an MTN office today to retrieve my lost sim, the newly retrieved sim is not functioning.

So sorry about your lost sim. This happened to me about 2 weeks ago, and below is the message from MTN Customer Care when I contacted them. They will also send you a sim swap...

unlink your NIN number from your SIM card.

Hi Aderant. MTN usually assume that the person that registered a line is the owner of the line. It will be assumed that the line belongs to your brother. You need to visit MTN...