if you want do use the preinstalled busybox-cron execute this command once after flashing

update-rc.d busybox-cron defaults


2nd solution: install vixie cron online which will start on bootup automaticly

opkg update && opkg install cron


possibly you have to transfer the existing busybox crontabs to the new crontab file after installing cron. You can check this with


crontab -l