Billing after cancelled services

I called to cancel my services to end on Sep 9 2021. After this, I got an email saying they were going to cancel my services on Sep 11, 2021. I called back twice to ensure that my services would cancel on Sep 9, and I was told by both representatives that my services would be cancelled on Sep 9, 2021. So by setting these dates, I should have had my last bill come on Aug 6, 2021 which states my billing cycle is from Aug 10, 2021 to Sep 9, 2021. After this, I got a bill from on Sep 6, 2021 for the periods of Sep 10, 2021 to Oct 9, 2021. Why would I be getting a bill for dates in which my services were supposed to be already cancelled? I called yet again on Sep 16, 2021 and was told that my bill would go away. I waited for it to go away and it never did. I was then billed on Sep 9, 2021! Why would I be billed for anything? I cancelled all of my services and used all of my own equipment.