Today I gave the ActivityPub plugin for WordPress another try, and was successful! First, I upgraded to the latest version (from 9.0.2 to 9.3.1). I accepted the default settings, so my new ActivityPub account based on my settings would be alsylves@andysylvester.com. Next, I logged into mastodon.social (https://mastodon.social/@AndySylvester), pasted my new account in the search box, clicked “Cancel request” on the old version, then clicked “Follow”, and saw the button change to “Unfollow”. Finally, I went back to my website, created a post, and saw it appear on my Mastodon home page. After that, I marked the post as a “favorite” and I created a reply to the post in Mastodon. Within a minute, it appeared as a comment on my WordPress post. Success!