Returned modem (have receipt), haven't been able to get charges removed
I returned my modem on 2/23. I received a receipt at the time of the return and an email shortly afterwords.
The modem rental fee was still on my account for my March bill. I assumed this was still on the bill because of the processing time and didn't think anything of it.
I was still charged on my account for the April bill. I contacted support and was assured that it was taken off and would be refunded for the billing periods I was erroneously charged a rental fee.
Again, on my May bill, the charge was still there. I contacted support again, and was assured that it was taken off. This time, the person would not state that I would be refunded for the three bills I have been erroneously billed, just said wait until next billing period to see if it charges were stopped.
I've already been charged for three months of a modem that's been returned, how can I check my account to make sure that the modem was in fact returned and that I will no longer be billed?