1. Mount "MountMe.iso"
2. Completely disable Windows Security settings and/or any other antivirus
3. Run "AUTORUN.exe"
4. Select all relevant softwares and click "Start installation"
5. The installation should occur and activate automatically. If not, make sure all antivirus is completely disabled. If the software fails to activate automatically, select "Activate Office' in AUTORUN.exe
6. Unmount "DVD Drive (D:) Microsoft Office 2021"

From KpoJIuK (RuTracker.org)