You have your Asus as 192.168.1.5 so if you set the Zyxel to 192.168.1.11 it will be within the DHCP range of the Asus.
You're also setting the gateway as 192.168.1.1 but there's nothing using that IP.
Why not just keep the Zyxel on the default of 192.168.1.1 ?
I always had my Zyxel as 192.168.1.1 and my Asus router as 192.168.1.2
Some others would recommend putting the router on another subnet like 192.168.2.1
Just noticed your screenshot. Why do you have enable WAN unticked?
I've made the suggested changes:
Asus IP address 192.168.1.2 (Photos 2)
Asus IP Pool starting address 192.168.1.2 (Photos 1)
Asus IP Pool ending address 192.168.1.254 (Photos 1)
WAN - 100% ticked. I double checked.
By using a laptop I was able to change the Zyxel to Bridge mode and revert back to the latest official stock firmware. Although the configuration setup for the previous owner was still present. I deleted them.
These are the default settings. Am I good to go with these? (Screenshot 3-6)