OS5 crontab problem

I’m trying to establish a rsync backup from my WD Mycloud EX2 Ultra running OS5 and a Synology.

I configured passwordless ssh and a manual rsync works perfectly.

Now I want to configure a cron job.

I edited

/usr/local/config/config.xml

and

/etc/NAS_CFG/config.xml

without luck. It will not survive a reboot.

I even did

mkdir /tmp/ubifs_flash

mount -t ubifs /dev/ubi0_0 /tmp/ubifs_flash

and edited

/tmp/ubifs_flash/config.xml

without luck also.

Any hint how to setup a cronjob?