Keep in mind that it uses the default phpBB cron. Default, phpBB will arrange for periodic tasks to be run automatically when users visit the board. When you have a board with few visitors (not counting bots) this may not be so regular as you expect.
You can instead choose to go to the ACP -> Server settings -> Run periodic tasks from system cron.
When on, phpBB will not schedule any periodic tasks by itself; a system administrator must arrange for bin/phpbbcli.php cron:run to be run by the system cron facility at regular intervals (e.g. every 5 minutes). This means the cron service will check each 5 minutes if there is a pending task to be executed, including (but not only) the feed fetching.