You may be able to activate it using your mobile phone using the ideas described here: https://www.techrepublic.com/article/how-to-activate-windows-offline-using-microsofts-text-based-opt...
I'm looking for a solution to activate Office 2021 retail editions(Home and Business or Professional) without internet access.
The key received from Microsoft has to be redeemed by adding it to a Microsoft account. Since about one year ago, maybe more, the redeemed keys were visible in the Microsoft account. However Microsoft changed something and Office 2021 and 2019 keys are not visible anymore in the Microsoft accounts. Office 2016(maybe 2013) redeemed keys are still visible in the Microsoft accounts.
With Office 2019 was possible to activate Office without internet connection with the redeemed keys, but now the Office 2021 retail edition redeemed keys are not visible anymore in the Microsoft accounts.
When you enter the received Office 2021 key on a computer without internet connection you will receive this error message: Sorry, we couldn't verify your product key because you're not connected to the Internet .
For LTSC or volume Office licenses the installer gives the option to activate by phone.
Any ideas are welcomed!