Hello.
Please read description of usage in plugin’s page and use it as per instructions.
I have. I installed the plug, set it, saved it, and have inserted the short code calls [read more] long text [/read] as instructed. Nothing happens, except [read more] and [/read] are displayed in the text. It appears that the plugin is not working.
The plugin is working pretty fine, here’s the backend page > http://prntscr.com/rsyfp4 and here’s its working online demo: https://apps.8web.gr/demos/wordpress-read-more-without-refresh-plugin/
Deactivate all of your plugins and retry please. Performing a clean new installation will also be helpful in order to verify that the plugin is working. If you are still unable to solve your local conflict, I advise you to talk to your theme’s author support area, since its more likely that you experience a theme editor issue.