banger
1 - login to UI of zyxel
2 - select networks -> vlan groups
3 - add group called bridge set vland id to 2, tick the include box on the lan port you want to use. (one used for wan)
4 - select networks -> interface grouping and assign bridge vdsl port to same lan port.
Note you may have already done steps 3 and 4 when setting up the bridge.
5 - now select networks -> home networking
6 - in group name box default is preselected, change this to bridge
7 - make a note of the ip address shown, change if you want to. (to keep things simple change it to 192.168.2.10 and make sure your old modem is not connected so no conflict)
You can now access the zyxel using that ip address on the wan cable, I didnt have to adjust my config on my router compared to my billion it just worked.