Miscellaneous topics that do not fit into specific categories.
Hello,
You cannot add a non-Microsoft branded device by using serial number. The serial number method is only for Microsoft's branded device, such as Surface laptop/tablet, Xbox, etc. Since yours is Asus, this approach will not work.
So, typically, if you sign into your Microsoft account in Windows, that should automatically add your device to your account. Lately, I am seeing a lot of delays showing up in your account. But it can be comsmetic.
Go to Windows Settings > Accounts > Your info, on this page, do you see your Microsoft account listed?
If you go to Settings > Accounts > Linked Device, is your device listed there?