Cancelled service

I ordered a router online which had a contract sim at MTN and it was delivered, I called MTN as I was unhappy with the plan they advised I should return it to the nearest MTN branch for cancellation which I did @ Maponya MTN branch, months later I start receiving text and messages from MTN demanding that I pay money I owe as I am in arrears which I was in disbelief, I call the call center and I was told whoever that was assisting me with the cancellation was supposed to give me a reference number which they never did, I’ve been to Maponya MTN several time and the Manager promised to get back to me which they never did

Please assist I need this matter resolved as it’s affecting my credit record