Can't Pay Bill for Closed Account

I recently moved apartments, and the account at the old address was closed. I failed to return the equipment in time, so I have a charge on that account but I am unable to pay it. I have seen the charge for siad account online, but when I tried to make a payment, it switched back to my current account at my new address, which has no charge currently. 

How can I make a payment on a closed account? Thank you.