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 didn't have to adjust my config on my router compared to my billion it just worked.
I am trying to get to stats page on 3925 in Bridge/Modem Only mode from LAN driven by Mikrotik router
So the setup is client (wired or wireless) --> Mikrotik --> single cable to VMG3925-B10B
From LAN client I can ping the Vlan2 interface on Mirotik, but not the home networking/group interface on Zyxel
If I add ie Eth4 to Vlan Group on Zyxel, then connecting directly on Eth4 I can access unit via that IP, so it does work on Zyxel (just not the two together)