A VPN will change your IP address so that the website you're visiting doesn't see your real IP address and aren't able to track you using your real IP address.
Your ISP still sees and knows your IP address that is connected to your modem that connects to them which allows you to connect to the internet.
So the ISP can still see your ISP and they can still see that it's you downloading the files or other data that you're downloading.
So while a VPN is good for stopping websites from tracking you it's does not work to prevent the ISP from seeing your real IP address.