In general it should be super simple to federate WordPress posts to the fediverse: Just simply install and activate the ActivityPub WordPress plugin and there you go.
I activated the plugin in two WordPress installation. In one it worked like a charm in one the federeation simply did not work. I aslo asked at the ActivityPub Support on WordPress.org (and checked Cron logs etc). Nothing helped.
Then, one day I found the solution: The Site URL / Domain name must be all in small letters. On this blog, I had the site URL = www.Locked.de. As soon as I changed to a small “l”, federation worked like a charm.