When two or more interfaces are configured on the WAN zone and configuring Transparent mode on an additional interface using the IP range from one of the WAN interfaces fails with error as seen below in screen shot:
"Error:Index of the interface:Transparent Range not in WAN subnet"
X1 IP address is 18.104.22.168
X2 IP address is 22.214.171.124
Configuring X4 in Transparent Mode with an IP from within X1 IP range of 126.96.36.199
You can configure an interface in transparent mode only using an IP in the subnet of the default WAN Interface specified on the Failover & Load Balancing Configuration.
To resolve/workaround this issue go to Network | Failover & Load Balancing and make sure the X1 interface is the primary interface set in the Default LB Group as seen below. If not, change this accordingly and then configure Transparent mode again on your interface. If configuring Transparent Mode with an IP address from the X2 IP range change the setting below to have the X2 as the primary interface.