When displaying ‘pop-up’ reminder window, keep in mind the pop-up work only when the calendar is the currently active tab. 

For the built-in email reminder that sends emails reminders (cron job required). See https://phpeventcalendar.com/docs/event-emailpopup-reminder/

To setup cron job, it is recommended to use cPanel Cron Job tool. See https://kb.iweb.com/hc/en-us/articles/230244168-Setting-up-cron-jobs-with-cPanel-. This tool is always included in host provider that uses cPanel.

When use PHPMailer, you are required set up an SMTP server in order to send out emails. See https://github.com/PHPMailer/PHPMailer