@Suhailmyself 

After you log into the router's user interface and click on the menu "Operation Mode" do you see an option "Range Extender"?

If yes, then I guess that is what others mean by "WDS support".

How exactly are you trying to change the router's IP address? Can you post a screenshot of the changes you are trying to make?

If tplinkwifi.net doesn't get you to the router's interface, then try to use the actual address that you've manually configured. For example, if you set the router's LAN address to 192.168.1.1, then use the link http://192.168.1.1

@woozle When I login and select range extender mode then select my existing wifi network whose range to extend. And by entering its password I complete the setup.So the router reboots and after that it neither logins with its primary IP address nor with tplinkwifi.net. And if I change the IP address of the router, the dynamic IP is not taken.So even the changed IP address does not login.

And during this whole setting, I do not see the option of WDS anywhere in the interface.

As you can see in this pic there is no WDS option

@terziyski Yes, I did the same thing but as soon as I select its range extender mode and do all the settings, after that it neither connects to my primary router nor will its user interface logging in again. Not from the IP address I changed and not from tplinkwifi.net

@Suhailmyself 

Try again like this:

- Restore the router to factory defaults.

- After that enter the router's configuration GUI, click "Quick Setup", select "Range Extender" and go through the steps until you get to the page where you configure the "LAN Type"

- Instead of the default "Smart IP (DHCP)" select "Static IP" and then enter an unused IP address within the subnet of your main router. For example, if your main router's LAN IP address is 192.168.0.1, then you could set the C20's IP address to 192.168.0.250 or if your main router's LAN IP address is 192.168.1.1, then you could set the C20's IP address to 192.168.1.250.

- In that same screen also set "DHCP Server" to "Disable", to prevent conflicts with your main router.

- Then Finish the Quick Setup and the C20 will reboot.

- Now check if the C20 has connected to your main router.

- If it still hasn't connected to the main router, then you will not be able to connect to the C20, as you already know. Because there is no DHCP Server at this point, you will have to assign an IP address to your client device manually. How that is done differs between Windows, Mac, Android, etc., just use Google search. Anyway, manually set the client device's IP address to one within the same subnet (similar to above, if your C20's IP address is 192.168.0.250, then set the client to 192.168.0.251 or if your C20's IP address is 192.168.1.250, then set the client to 192.168.1.251)

- Now you should be able to access the C20's GUI via http:/192.168.x.250 and you can look for clues why it doesn't connect to the main router, for example, check the System Log.

@woozle 

I also tried this method but the still same problem or maybe I am not able to connect

As soon as I set the IP address to the subnet of my current router and disable the DHCP server when the router is restarted, neither the internet is connected nor the user interface of the router is logged in.

I am using windows 10

I thought maybe there is some problem with my router so I got it replaced but still, i am unable to use extender mode

please help me.

@Suhailmyself 

If you have got to the point in my previous post when the C20 is configured, but doesn't connect, then you have to manually assign a static IP address to network card of the device you want to use to log into the C20, in your case a Windows 10 PC.

Here is a guide how to do that: https://www.tp-link.com/us/support/faq/919/

You have to set a static IP address that is within the same subnet as the C20.

For example, if you have chosen 192.168.x.250 as the C20's static IP address during the Quick Setup, then maybe use 192.168.x.251 for the Windows PC. For the subnet mask enter 255.255.255.0.

You can leave all other entry fields empty for now.

If done right, then you should be able to get to the C20's user interface by entering the direct link with the C20's IP address (http://192.168....) in your Browser and look at the C20's System Log.

P.S. Once you are done with the troubleshooting remember to set the Windows 10 PC back to "Obtain an IP address automatically" and "Obtain DNS server address automatically".

By the way, what are the security settings of the Wi-Fi of your main router? Like, WPA, WPA2, TKIP, AES this kind of stuff.