Worst service experience

I called to give my 30 day notice for cancellation. The lady told me the cancellation will only be effective after 30 days. Then they immediately cut my connection and told me that they moved my plan to prepaid effective immediately, which is not what that lady told me. And they tell me they will still bill me the full amount and there's nothing they can do about it. I'm now inconveniently left with no wifi connection in the middle of a work day and will be billed the full amount because of this agent's mistake and they are unwilling to take accountability.