I am not sure I am setting up the LAN1 interface properly as per those instructions I linked... as soon as I reboot the modem and router when there is a cable from the Zyxel LAN4 to my switch, the Zyxel IP address (192.168.1.1) appears in the wifi from the ASUS router (192.168.1.254) and I cannot access the internet.
At the moment, cables are Zyxel LAN1 to ASUS WAN and Zyxel LAN4 to ASUS LAN (via switch).
On the Zyxel I was able to go to Network Settings > Interface Group and create a new group called Bridge with VDSL used and LAN1 assigned. I rebooted both devices and go the problem. As soon as I unplug the LAN to LAN cable and reboot, it is fine but I have no access to 192.168.1.1