btw:
consider creating a child theme instead of editing your theme directly – if you upgrade the theme all your modifications will be lost. Or create a plugin with the code above.
I see. The problem is some (or all) of those page templates don’t display post content (as written in a content editor). That’s why this plugin does not know where to append the additional content.
This plugin depends on templates to use the_content() function to display normal post content. I’m sorry, but there is not much that can be done about this.