You should have remembered your expiry date and notified them before that. Most ISP automatically renew your contract in hope that you forget to cancel!
I had a smilar situation, except that I notified them 2 days ahead of my expiry, but they still charged me $407, I had to email them back and ask for refund. So just write them back at
accounting@acanac.com, they will instruct you how to return the modem to them.
Good luck!