Good evening @Giftiaa,
Welcome to the Community!
Have you had a chance to download the PDF version of your invoice to see what services are the charges corresponding to?
When did you send back the modem? Did you keep the tracking number to check the status of the return?
Based on the information you've shared, it sounds like the payment you made on December 4th would have been for the November invoice. The last bill typically states "Final Invoice" on it.
In order to verify the charges for you, I would have to access your account. Please send us a private message @CommunityHelps for further assistance. You can check out the How to send a Private Message blog post to learn more about our PM system.