Cancelation Issue - Being billed after contract end-date

Had mobile LTE wifi. Constant slow connections and issues. HUGE data consumption.

Then they phoned me during my upgrade period (2months before the contract finished), and I asked to cancel my contract upon the contract finishing. The service provider then said no problem he will send this off for cancelation when the contract date has ended. I then got a call closer to that date again asking for an upgrade and I said no I want the contract canceled upon the final date. And again got a confirmation that it will be canceled.

Turns out, service providers cant cancel contracts and that needs to be done by me as the client. But here's the catch, you have to give 30days notice for cancelations. So only upon me receiving an invoice for the month after the contract end date did I inquire as to why Im still being invoiced and only then did I get the correct info about how to cancel when i could have done this 2 months ago and wouldnt be re-billed after the end date