It does not work

I tested it (Version 2.1.0) on a test site running on WordPress 6.6.1. It did not work. I deactivated all the other plugins, but it still did not work. The theme was the official Twenty Twenty-Four Version 1.2.

I tested it on another test site running on WordPress 4.9.26 with Twenty Seventeen. As it is not claimed to be compatible with WordPress 4.9.x, it naturally did not work after deactivation of all other plugins.

So, I deactivated this Missed Scheduled Posts Publisher on both of the test sites, and then installed another plugin called Scheduled Post Trigger. That one works in a different way (it publishes missed scheduled posts when someone visits the site), but it worked.

You had better use that Scheduled Post Trigger, or just write a simple command line as a cron job (the path to the location of the PHP version you use plus a single space and then the path to the wp-cron.php in your WordPress) on your cPanel or any other control panel as long as a cron job is available on your server.

This server cron job probably works, and when it does, it works the way Missed Scheduled Posts Publisher works. It publishes missed scheduled posts not at every 15 minutes but at an interval you specified, like every minute, every five minutes, every 15 minutes, every 30 minutes, every hour, etc. That means it would be more versatile.