I've been largely disappointed with my attempts to use ActivityPub with Wordpress. I do revisit it every now and then.
Because of how federation works, you will not see any old posts. There is no mechanism for pulling up the history of posts and displaying them.
That's not true. Reading the ActivityPub outbox is the way to do that. Mastodon doesn't and I'm not entirely sure why (though I could probably find out).
I have also had poor results trying to use the Friends plugin with ActivityPub. Incoming replies result in high CPU usage for many seconds, and outgoing replies didn't show up last time I tried it.