You could try to use Omnias WAN Port as your trunc port (use eth2.XX in the bridge config instead of Lan2.XX). You may then configure a LAN Port (e.g. LAN4) as your WAN port in WAN Interface.
Thereby you would bypass Omnias internal switch for your vlan config. Maybe this helps.
I don‘t want to blame the internal switch for not working with vlans at all, I just had some strange traffic problems with it by myself using vlans on it and this workaround fixed it…