Tools for Office 2021 Pro Plus LTSC
1. Download ZIP from the link above.
2. Unpack archive, open Configuration.txt file with Notepad.
3. Edit text inside according to your needs as discribed in this post, copy your Activation Key instead XXXXX-... 
If you don"t have a key, you can remove PIDKEY="XXXXX-XXXXX-XXXXX-XXXXX-XXXXX" string. 
Close Configuration.txt file, saving changes.
4. Run START.cmd file and wait for Office to install.
5. Done
Note: If you entered online-key, Office would be auto-activated after installation.
If you entered offline-key (for "phone-activation"), after installation open any Office App (Word, Excel, etc.) and choose "Phone Activation" in pop-up window.
If you entered "dead", but non-blocked key, after pop-up window opening choose "Phone Activation" and call MS Support.