Time Machine backups to Time Capsule use internet data. How to stop this?
Solution 1:
Time Machine is not accessing the internet. Your ISP or your modem are incorrectly categorizing the traffic as network traffic.
Time Machine operates entirely between your computer and the drive itself (necessarily communicating to the device the drive is plugged into). There's no intermediary servers.
You will need to pursue solutions specific to your modem or ISP.