the Turris Onmia is configured as an ethernet switch and one-armed router behind a Fritz!Box 7490 (Fritz!OS 7.01). br-lan is configured with static protocol and Use builtin IPv6-management enabled. br-lan gets a fe80::/64 address but unlike all other hosts no public dynamic 2003::/64 prefix.
See here, I believe you are in the same situation:
Edit: You configured ipv4 static, I am not sure if this sets ipv6 to static, too. dhcpv6 is needed for prefix delegation. Maybe someone else can answear this. What definitly should work is to set a static ipv4-dhcp lease on your Fritzbox for your TO and then set complete interface-config on TO to dhcp.
EDIT: I just saw that you don’t have a typical WAN/LAN setup. In that case you can create an additional interface in LUCI named eg. “LAN6” with protocol “DHCPv6 client” and as physical interface a new userdefined interface named “@lan”