We are using this plugin on a regular basis on this site so we know for a fact it does work, including with Joomla 3.9.1 which is Joomla current version as of today.
Please note that a resend button is only displayed for users who need it (ie non activated users). Which means that on most sites, you will never see any button as users normally validate their accounts.
Things you can do to test is as follow:
1 - be sure to use the native Joomla admin template. Any other template can kill the CSS of the buttons and hide them.
2 - Clear any Joomla cache
3 - Check your PHP error logs for any error
4 - Check your browser console for any error
5 - Disable one by one plugins that are running in the admin and could interfere.