Avast wifi scanner - "device is accessible from the internet"

Hi. My parents’ computer has recently been showing an error when scanned. apparently their router is accessible from the internet now? They’ve been using it for a while and this error never came up before so I’m not sure what to do. They’re not really interested in installing a VPN unless absolutely necessary. (I can post a screenshot of the error message with all of the specifics if necessary but I’m not sure if any of that is sensitive information?)

Thanks

. "device is accessible from the internet"
Does router have default admin name and log in password?