Poort internet provider

Pay account every moth. Had to stop debit order Because my pay day date has chengesd. Let them know that I will EFT but ot it will be late. Payed on the date I said i would and did so every month after that but still my line got suspended because of non payment yet there is not one month i did not pay, so where did my installment go to that I paid. Shared with them thw proof of payment as they asked but still they argue that i am behind.