How can I get my Synology DiskStation to update IPv6 prefix?

Hi everyone,

i would like my Synology DiskStation DS216j to update my Zone. So installed the DDNS Updater 2 on it.

At the moment, only my IPv4 gets updated. The IPv6 (prefix) does not get updated. I attached screenshots showing my settings. On the second screenshot you can see the “Use IPv6” checkbox. Even if it is selected, the IPv6 won’t update.

Can anyone help me, what I did wrong and what I need to do, to get my IPv6 (prefix) updated?