The WAN-Connection of the Firewall must be routet to the Zyxel Router.
The IP Range of the Zyxel and the Firewall must be different.
Set the WAN IP of the Firewall to one of the IP-Range from the Zyxel.
Set up a Gateway IP for the Firwall.
example:
Zyxel has external IP 195.145.###.###
Zyxel has internal IP Range 192.168.2.###; Gateway IP is 192.168.2.1
Firewall WAN = 192.168.2.250
Firewall LAN gateway = 192.168.3.1
Firewall LAN IP Range = 192.168.3.###
