As I wrote, you have only two options: Try with another cable or use different port on the device before the C20 router. That usually happens when the cable is faulty or too long or one of devices'...

@bobred Use this guide for newer devices. Your setup is looking good - just set the wireless security of the C7 (TP-Link_Extend) to WPA2-PSK / AES (currently on Auto). You may need to reboot the...

If you can't find such an option anywhere (it's usually in System Tools->System parameters menu) then the feature to change the negotiation speed is not implemented - you can only observe the already...

(A) No (B) Yes, that way you could avoid the potential wireless interference between the two APs. (C) Yes, same advantage as (B), but in that case all the AP2's wireless clients have to support 5GHz...

@davegray According to your screenshot and to your device manual 12.2.3 you have to plug the cables in ports LAN2 & LAN3 on the router side.

@Vaclavpe Probably you won't see the extended router as a client of the main one. It's enough to check that on the extended router - " Now you can go to Advanced > Status > Wireless to check the WDS...