http://www.mywifiext.xyz/netgear-ex6100/ is a default web address to open up the setup of Netgear wifi extender settings. But sometimes the page is not showing any setup page, instead, you see a website with a normal page or an error on the page. The error is shown either because you are not connected to the network or your wifi extender is not connected to your main network. Check your network cables if they are connected to your WiFi Modem correctly. Check if your modem is working good. All the lights (at least 4) are glowing. If all are good, try once by switching Off the modem and Turning On again and try connecting your extender again.
When you are connected to the network you will see the MYWIFIEXT web page like above. If you are not seeing the same page on your screen then you need to check your network.