Nighthawk AC1900 Range Extender won't allow wifi connections

I've never been able to get the WPS setup to work. So, every time I've tried to set this piece of equipment up, I've used the manul setup, connecting the thing to a PC via NETGEAR_EXT wifi.

I can make it all the way through the setup process, but when it gets to the screen where it shows the network info, and says "To continue the installation process, connect your device to one of the above networks, select this checkbox, and click the CONTINUE button", I am unable to get it to work.

To be more specific. I've tried to use "One Network", but that didn't work. I've also tried to set it up with different SSID/password. This also didn't work. When I try to connect to the new extended network, it always fails. I've tried from a Roku, 3 different android phones, 2 laptops, and 2 desktop PCs. They always fail. All it says is "Can't connect to this network", on Windows machines. On android phones it just says "Connecting...", then fails. No error or message, it just stops. Then it tries again. And stops. And tries again. And stops. Eventually, after a dozen tries, it just says "Authentication failure". I have verified 1000 times over that I have the right password, and it is showing on the setup screen, so it's easy to verify. 

It DOES see the SSID, from windows machines and android phones. The nighthawk is clearly broadcasting something. However, it just won't let me connect to it.

If I just check the checkbox on the setup screen, and continue, the nighthawk WILL give a solid internet connection to anything connected to it via ethernet cable. So I know it is connecting properly to the router. And I know it can at least broadcast an SSID. However, it does not accept wifi connections.

What could be wrong, and what could I try to fix it? Right now, it's just a really, really expensive access point, instead of an actual range extender.