C9 is Gigabit Router, the ports on it are all 1000Mbps, I think it can handle almost 1000,recommend 500/500

DHCP if off, you can unplug other cables from the router, just a PC to the LAN port, set a static IP on the PC be the same segment with the router and login

Suggest you simplify the network topology, unplug all the cables from the router (except power), just one PC to the LAN port. power off your extenders if you have If still not work, call the tp-link...

It goes back to the default 192.168.0.1 this IP address? Any error message When you click on save? Maybe you can try to change another browser or use another device to login and change the LAN IP

the_hamster wrote Finally, success! Used the C7v2 as primary router (provisioned with IPv6 addresses, etc,), then set up the C7v4 as "AP" extender. Got around the seemingly intractable IPv6 issue w/...

vieldcs wrote I see the notice here , you can also have a look : http://forum.tp-link.com/showthread.php?101094-Security-Flaws-Severe-flaws-called-quot-KRACK-quot-are-discovered-in-the-WPA2-protocol...