MTN and Mondo

I took a Wifi contract in October 2021 from MTN, only to find a 3rd party called Mondo is responsible for providing me with router. On delivery, the router was faulty, it could not charger. I returned in within 10 days. I am still being billed for the router that never worked. It's a different contract from my cellphone contract but MTN still blocks my phone and demand a payment for the Wifi. All of a sudden, I must sort this with Mondo....I am really being taken for a ride. I have been doing follow ups since December 2021 until this day.