Mondo cancellation department

I was contacted by an agent from Mondo on 10/06/2025 offering me a data deal and I agreed to it. The router and sim were delivered to me the following day and after a few days I realised that I do not need the data. I then called to request for a cancellation as I was advised that I am allowed to cancel within 7days. Collection was done by DSV after a week and some days in that very same month of June. I was billed by MTN in July and now in August for this deal and they say it’s because Mondo hasn’t sent them the cancellation notice yet. When I call Mondo they tell me their cancellation department hasn’t loaded the cancellation to date. This is absolutely unacceptable and horrible of Mondo. How long should I keep paying MTN for something that I cancelled and never used?