Only specify IPv6 suffix in DSM

I'm using a dual stack connection at home and have a DS213+. As my public IP (both v4 and v6) is dynamic, I'd like to only specify a suffix in my Diskstation's network configuration. My router supports DHCPv6, offers IA_PD and IA_NA.

My ISP delegates a random /64 to my router each time I connect and I want my Diskstation to use this prefix plus a static suffix. I did NOT find any option to do this.