Backup WAN
Use this screen to have the LAN/WAN port on the Zyxel Device ports panel to act as an Ethernet WAN port. The Ethernet WAN connection has priority over the DSL WAN connection.
It’s not enough to just enable the fourth LAN port as a WAN port here. You must also go to Network Setting > Broadband screen and create a new interface for it with the Type as Ethernet and Encapsulation as IPoE. It’s suggested to enable NAT.
*The Ethernet WAN connection has priority over the DSL connection. See WAN Priority for the Zyxel Device WAN priority.
Click Network Setting > Broadband > Backup WAN to display the following screen.
Network Setting > Broadband > Backup WAN
The following table describes the fields in this screen.
Network Setting > Broadband > Backup WAN 
Label
Description
State:
Click this switch to enable backup WAN to have the LAN/WAN port act as an Ethernet WAN port. Otherwise, the LAN/WAN port remains as a LAN port.
Cancel
Click Cancel to exit this screen without saving any changes.
Apply
Click Apply to save your changes.