I see that your ISP provides the following on the WAN6:
- IPv6:
2401:4900:1c28:9fff::19f:2188/128 - IPv6-PD:
2401:4900:1cc8:f24a::/64
With IPv6-PD available (great news!), you should be able to assign the delegated IPv6 range "2401:xxxx:xxxx:f24a::" to any LAN interface. Since this isn't working in your current setup, try resetting your OpenWrt router. Check if the delegated IPv6 block is automatically assigned to the LAN interface by default with the standard OpenWrt configurations (it should!). If it is, you can then proceed with VLAN configuration, and it should still be automatically assigned to any available VLAN. Once you confirm automatic assignment, you can manually assign it to the desired VLAN interface.
After that, you can proceed to relay your WAN6 IPv6 range "2401:xxxx:xxxx:9fff::" to the other VLAN interfaces, and I think my setup here is similar to your: Relay a single IPv6 /64 prefix into several VLANs - #10 by alexq