I'm still not clear why you need to set the Zen IP as fixed in order to use the block of 5. The Zen IP will never change anyway. I have a Zen connection with 5 usable public IPs and it works fine with "not fixed". The first Aux WAN IP in the WAN IP Alias table Is the public IP address of my router. Although I accept that I am using multi-NAT it seems that Draytek say you can use a combination of these things.

Are you following the "Dedicated LAN Interface" tab on this page? http://www.draytek.co.uk/support/guides/kb-drayos-iprouting#2-dedicated-lan-interface