Your printer and your computer must be on the same network and they must be able to see each other without restriction. In a lot of public networks, this ability is restricted by implementing "device isolation," which means your device can get online, but it can't see or communicate properly with other devices on the network - this is to prevent unknown or unauthorized users from accessing someone else's information (this is a networking protocol, not a Canon feature).
If the network(s) that you have access to do not utilize isolation, and you are able to access *all* devices on the network, as well as add and remove devices on the network, then you'll want to completely reinstall the software for your printer onto your computer. Once a printer is paired with a computer, the computer remembers the connection until you tell the computer otherwise. Without overcomplicating it, the easiest way to handle this is to reinstall the software like it's brand new. Head over to https://canon.us/ix6820setup and follow the steps. It will uninstall anything you have on your computer (old pairings), and reinstall the software and drivers so you can pair it to your current location. You'll need to do this whenever you change the connection on your printer - whether it's to update the WiFi connection, or change from WiFi to Wired/USB. It's fairly straightforward, but if you get stuck, let us know, or reach out to support at 1-800-OK-CANON (800-652-2666), weekdays from 9am to 6pm (ET).
