Refer: https://askubuntu.com/questions/1150144/spell-checker-not-working-in-libre-office-6-0
File: dict-en-20200601.7z
This worked for me. Even hunspell or myspell, as suggested by resplin, didn't work.
However adding this extension to Libre Office worked like charm: https://extensions.libreoffice.org/extensions/english-dictionaries/
You'll need to go to Tools > Extension Manager > Add > {upload extension file}. Restart.