have a look here: https://wiki.phpbb.com/PhpBB3.1/RFC/Modular_cron
Use system cron
In order to use the system cron you need to be able to set up cron jobs on your server.
First of all, enable the 'Run periodic tasks from system cron' setting in the 'Server settings' tab of the ACP.
Now you need to set up a periodic cron job. This depends on your system, but you usually have to edit /etc/crontab and add a job that runs every minute in the following format.
* * * * * cd "/path/to/board"; ./bin/phpbbcli.php cron:run
Note: The 'cd' is needed because all paths within phpBB are relative.
so if you have access to cron jobs on your server put something like
*/15 * * * * cd "/path/to/board"; ./bin/phpbbcli.php cron:run
in your crontab or create a new file in /etc/cron.d and every 15 minutes phpbb cron is running and will check your rss feeds.