WAN Port Inactive / IP Issue Behind ISP Modem

Zyxel_Tina

Q: Why does the FLEX H WAN port stay inactive or fail to get an IP from the ISP modem when replacing an old router, and how to troubleshoot it?

A: When swapping to a new USG FLEX H behind an ISP device, connection failures issues typically stem from ISP-side binding or configuration mismatches. You can check and troubleshoot the following directions:

1. Resolve ISP MAC Address Binding (Most Likely):
   1. Option A (MAC Cloning): Go to Network > Interface > Interface, edit the WAN interface, enable Overwrite Default MAC Address, and enter the old router’s WAN MAC address.
   2. Option B (Reboot Modem): Power off the ISP modem for a few minutes and turn it back on to clear the previous MAC lease.
2. Set WAN Port Speed Manually:
   • Go to Network > Interface > Port and set the port speed to a fixed 1 Gbps, or to the speed that matches your modem’s port, instead of Auto to rule out auto-negotiation problems.
3. Hardware Cross-Check:
   • Replace the Ethernet cable or connect the WAN port to a local PC/switch to verify if the port LED lights up, confirming the port itself is hardware-functional.