Ok. I think I´ve got it.
It seems this feature is only available with Office 365.
I´ve installed both, Office 2021 (already installed) and Office 365 (new install), set my personal account on both suites and, like magic, the checkbox appeared again.
I´m not involved with Microsoft 365 Insider program so this shouldn't be a must for you guys.
Let me know if you have the same luck because I lost almost half of my day looking for a solution for this #$%&! including scrolling forums and updating my windows too!