Synology Community
https://community.synology.com/enu/forum/1/post/13010907.11.2020 · There is no support for updating DNS servers with IPv6 only as there is no __IPV6__ option or the option Set External IP is only set for IPv4 only. ... One minor problem is, that in Synology DDNS windows IPv6 will still remain empty, as if it's not working, but if you check at noip.com you'll see that it's been Updated.