There is 2 parts
The above is in Discourse user preferences. If they chose not to they may need to enable in mobile browser?
After adding to home screen (android) may need to use android preferences to ensure notifications.
You want folks to use pings though like @members. An @group might be better. Though notifcations pings may still have delays vs a chat ap.
In Discord some use a bot script to poll posts from topic tags or users. Not sure if whatsapp has that or not.