If you want to cover more local networks (guest, lan, dmz, lxc containers, …) with IPv6, you need to get bigger assignment. Prefix with the length 64 is sufficient to address only one network.
Prefix with the length 48 can be used to address 65 536 networks, or you can split it to longer prefixes and route them over VPN, for example. Up to your imagination.
So, ask for the routed /48, on Omnia update the configuration of the tunnel with the routed /48, apply the settings. In LuCI fiddle with the assignment length of guest and lan interface (set it 64 for guest and 60 for lan), apply the settings again and check that you have the IPv6 connectivity on your lan devices.
I suggest to check out the IPv6 book available for download (CZ only)