I don't know if it is Router (ER8411v1 firmware v1.2.0) issue or Software Controller issue but DHCPv6 doesn't accept the IPv6 local address range fc00:: or fd00:: as you can see here - 

But it accepts the documentation/example IPv6 range (2001:db8::)

I need to use DHCPv6 because my ISP only provides /64 prefix. This means that I can only use SLAAC+Stateless DHCP for a single VLAN. For other VLANs, I have to use DHCPv6 to communicate. One of the main reasons for this is to set AdGuardHome DNS, which is available on a different VLAN from my home devices.