WordPress Cron Jobs are not “real” Cronjobs, instead they relie on somebody visiting the site, then looking if it’s time to do scheduled jobs.
1.: Someone visits the website
2.: WordPress calls the file wp-cron.php
3.: wp-cron.php looks into the database if there are scheduled jobs that are due at this time.
4.: wp-cron.php does all of the scheduled jobs.
If you want to make sure that your cronjob is executed on time, you need to do some more steps:
1.: Schedule your cronjob. You did that already.
2.: disable the integrated WordPress-Cron. Edit your wp-config.php and insert
define('DISABLE_WP_CRON', true); below the line
3.: Create a “real” cronjob on your server, set to run every so often (5 Minutes, 15 Minutes, 1 Minute, something like that). I don’t know if and how you can create a cronjob on your hosting, that’s something you got to ask your hosting company. Make the target of your cronjob yourdomain.com/wp-cron.php
4.: Success! Your cron should work now as intended!