Cannot log into EX6200 configuration page

TL;DR You need to be connected to the extender's Wi-Fi network in order for http://mywifiext.net or http://www.mywifiext.net to work.

You have to be connected to the extender's Wi-Fi network in order for either URL (http://www.mywifiext.net and http://mywifiext.net) to work. Otherwise, you will get the helpful "Oops!" page, which actually comes from a server on the Internet that Netgear was nice enough to set up, that tells you the same thing (bold emphasis mine): If you are looking to connect to the user interface of your NETGEAR Powerline or WiFi Extender device, please connect to that device’s WiFi hotspot and try the URL again (http://mywifiext.net).

The reason you have to be connected to the extender in order for the URL method to work is that the extender intercepts the DNS lookup for mywifiext.net or www.mywifiext.net and directly responds with its own IP address. If, instead, you are directly connected to the router's Wi-Fi network, then the DNS lookup will actually be sent to the DNS server configured on the router. Unless you are running your own DNS server, that server will be on the Internet and will return the IP address of the server that has the "Oops!" page.

On the off chance that you are actually connected to the extender's Wi-Fi network and the URL isn't working, the extender does have its own IP address. By default, it will have obtained one using DHCP. You should be able to find the IP address listed on the Attached Devices page in the R7000. 

You can open a Command Prompt and type nslookup mywifiext.net . If you are connected to the extender, you should get back its IP address. If you are not connected to the extender or if the extender accidentally forwarded the lookup to the router and onto the Internet, then you will get back the IP address(es) (54.149.149.202 and 52.10.113.39 when I checked) of the "Oops!" page.