Before I say anything else, yes, I have checked all the boxes on my posts to ‘allow comments.’

Two places?

1. Settings -> Discussion

2. Where you create/edit posts or pages in admin, check if you have ticked comments for your posts/pages in the “discussion” box below the editor. If you don’t see the “discussion” box there, in the ‘screen option’ dropdown field (located near the top right), there are tick-boxes to show the ‘discussion’ box for the posts/pages. Tick mark it. In the ‘discussion’ box below the post/page editor, you can then allow/disallow comments for the page.

If you can install plugins, install “Health Check”: https://www.ads-software.com/plugins/health-check/ On the troubleshooting tab, you can click the button to disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site.

So, thanks for bringing my attention back to that page, anyway!