Does your cellphone detect your main router's wifi? Make sure the wifi is enabled. Is it too far away from the extender? How about move it closer to the RE?

Do you mean you just need all your devices connect to the router's wifi but no need to access internet? If yes, the router should make it. It will broadcast Wi-Fi and your devices can connect to them...

Try to press reset button for more than 10s to hard reset it. Then, try to reconfigure it to see whether it can get back.

Hey, does your PC use win8.1 OS and above? It requires the OS should be above win8.1.

Have you changed other ethernet cables? Or change another port on your main router?

You can disable Broadcat wireless SSID on the router. After that, you can still receive wireless but won't detect wifi SSID any more.