It is very easy to log in to extender.linksys.setup page. Here are the steps that you need to follow
Turn on your PC or laptop.
Pull up an internet browser on it.
In the address bar appearing at the top-right side, type extender.linksys.com and press the Enter key.
You will be taken to the Linksys WiFi range extender login web page.
Enter the username and password into the given fields.
Once done, hit on the Log In button.
Please read the originial post here: https://extenderlinksys.live/