Cancel my cancelation

I cancelled my subscription last October 2020 and then they told me its gonna be cancelled 30th November since its a calendar month notice , okay fine November came and my wifi was still active, got an invoice which they billed me 5000 for cancelation okay I complained even sent my statement as I paid every month. Well nothing happened my account was active until yesterday. I went back and asked them to cancel my cancelation and send me the correct invoice I was sent from pillar to post...my kids need to do assignments and researches and no one is willing to help. I dont know what to do now