MTN is useless

I took out a home wifi router in May 2024 and it wasn't working for me so I decided to cancel it around July/1st week of August by going into the store and do the cancelation.. They told me that it was canceled and I'll receive an email, however I found out that they didn't do the cancelation at that time it was only done in December and now I owe MTN.. I want MTN to sort this out coz now my credit score is low due to them being unprofessional. I want to take this further and let everyone know how useless this people are.