You should set up a cron job on the server to do what the plugin authors are asking you to do: execute the php command they’ve given you. Just make sure you’re not setting up WordPress’s wp_cron(), since this one is not a real cron job. From this article:
When it comes to WordPress, you can schedule an event to happen at a
certain interval, but it doesn’t operate like a classical cron job.Instead, the event is set and scheduled and written to the database.
The next time a user hits the site, the WordPress cron system will
look to see if an event is scheduled and, if so, will then fire the
event.Notice the problem?
Someone has to visit the site before the event actually kicks off. So
if you’ve scheduled something to happen hourly, but no one has visited
your site in the last hour, then the event will never kick off.