@jpkan 

Hi,

Please try to log into the Archer C2600's web interface using a web browser and see if that works. ( http://tplinkwifi.net or http://192.168.0.1 )

By the way, after you performed the factory reset, did the names of the router's Wi-Fi networks change from whatever customized names you previously had configured back to the default names printed on the router's label? (something like TP-LINK_1234 and TP-LINK_1234_5G)

  @woozle 

Thank you woozle for your prompt answer

1-yes after performing the factory reset the SSID change to the default one and my phone on which tether app is, was connected to this new SSID without any problem. When I use the tether app, it recognizes the router and ask me for the username and password. If I use the default one it says invalid username and password ….

2-I've never succeeded logging in this router with the web browser (firefox, edge ….) with my PC…. And nobody could …… even local TP link customer support …. There is an issue of compatibility between my windows language (I'm French) and the router internal software language (English – I live in Malaysia). Even changing my windows language to English it won't work. When I say it doesn't work it means that I can access the router but he asks me only for password never asks me for username ….. So whatever password I key in nothing happens not even invalid password notification nothing happens … that's why I was using tether because it worked all the time even in French on my phone

3-So I had to go to a local friend (in Malaysia) to use his PC to access the router. Due to English compatibility it worked at last ….

I still don't know why it didn't work with tether but anyway …..

So at last it worked …. thanks…. I was able to change the settings of this router

  @jpkan 

Hi,

TP-Link's router models for service providers (like your EC440-5Gu) usually are not supported by the Tether app. That the EC440-5Gu is even shown by the Tether app is likely a bug.

What is the device you have the Tether app running on? Did the web browser of that device also not allow you to log into the EC440-5Gu?