Question about Zyxel EE5301

I would disagree with digitecs answer because as soon as you use the SFP+ Module from the ISP it replaces the built-in Ethernet WAN Port in the routers configuration. You still have only the connection types "GPON" and "Ethernet" in the WAN connection options and the "Ethernet" settings apply to the SFP+ port. Only if use "PON" then Wan port could be configured separately, but I don't know how "PON" can be used with the EE5301, but "SFP+" will be "Ethernet" if plugged in. I already have have this setup running here. So you don't have the possibility configure the WAN Port if you're using SFP+.

The WAN port on this router cannot be directly used as a LAN port by default, but it can be configured in bridge mode to function differently depending on your network setup. Since your ISP connection comes via the SFP+ port, the WAN port is not needed for WAN and may be repurposed through configuration changes. Specifically, the WAN interface mode can be set to «Bridge,» which may allow the WAN port to act as a LAN port or be bridged with LAN ports, depending on the device's firmware and settings.

The router has one 10GbE WAN port and multiple LAN ports (one 10GbE LAN and two Gigabit LANs). The SFP+ cage is designed for flexible fiber connections, so if you use the SFP+ for WAN, the Ethernet WAN port can potentially be bridged or reassigned to extend LAN functionality, but this requires manual configuration in the device's network settings.

In summary, while the WAN port is primarily designed for WAN use, it can be configured (e.g., set to bridge mode) to serve as an additional LAN port when the SFP+ port is used for WAN. You should access the router's management interface to modify the WAN interface mode accordingly.