If the user can't download it from the Mac App Store, they might need to use the "Install macOS Catalina" app via a different method. Some sources suggest using the -seed or developer account options, but that's probably not relevant here. Alternatively, there are third-party tools like UTM or others that bundle macOS ISOs, but I should caution against piracy or illegal downloads.
Wait, but if they already have the DMG, they can use Disk Utility to mount it, then run the installer. But if they need the DMG itself, they might need to find a way to get it via the App Store first. So maybe the user is confused about the process.
First, they mentioned "clean installation," which is different from an upgrade. So they probably want to start fresh, not keep their existing macOS setup. That means they might need to create a bootable USB drive using the DMG file or the Install macOS app from the App Store.
But wait, Apple doesn't provide direct DMG links anymore for macOS versions. They usually distribute through the App Store. For Catalina, you have to download the installer from there. However, sometimes users have issues downloading through the App Store, maybe due to network errors or app store changes. They might be looking for alternative ways.