MTN WiFi charging me

I have been sending mtn emails to cancel my WiFi that was a contract for 12 months. After 12 months they changed it to a month to month contract, I don’t know who asked them to do that because I took another contract after the first one ended. Now they are charging me every month. I sent an email asking them to please cancel and they said I just go to mtn branch and return the router and I did. I asked them at the branch to cancel and they said they’ll give me IRM to request for cancellation. I did that and someone came back to me saying I must visit the branch. Now there’s a back and forth. No one is assisting. I am a frustrated client