ACT Fibernet — Billing and service

I am on a yearly plan for Act storm and my bill was due on 15th July. Since their service had started to deteriorate I requested for a change of plan to monthly on 10th of July. They never responded but sent threatening messages about disconnecting the internet. To avoid disconnection I paid the entire years bill of Rs16779 on the 14th of July.
To my surprise on 24th of July they changed my plan to monthly without even reaching out to me or checking the billing status. After lot of calls and requests it was changed back to yearly. Now I have been sent a bill from July 1st to July 29th of 1129 when my previous plan was active till 15th. And the change of plan to monthly happened only on 24th that too because of their mistake. And I am expected to pay for it. I'd rather cancel the connection and go with some other provider than tolerate such pathetic services.
Helpdesk is like a parrot. They don't want to know what is the issue. All they respond is "pay your bill" when I have paid for an entire year.