The simple reason for it might be the weak physical connection between your
WiFi router and the WiFi extender. As it is a local web address, it might
cause errors if one is using intermittent or spotty wifi.
On the other hand, another reason could be a typo mistake. The address entered
on the address bar may be incorrect or the typo can be incorrect. And also, if
you are using a Mac Book, then mywifiext.net will not work. For mac, the
address is https://mywifiext-local, which is used for login.