I got you, I'm sorry for that. Currently, there are only web wrappers, like ZapZap. There have been clients, but when Meta finds them, it will ban you and try to block the app.

maybe you can look into beeper. haven't used it, but as i understand it's a matrix server with all sort of bridges. I use the deposited version which works fine (at least as much as whatsnot allows it)

edit: just saw that somebody already wrote details about mautrix, which would be the self hosted solution.