@Kuzey_Ext Hello, 

Please verify your Wi-Fi password first, and factory reset range extender and set up again, It is recommended to set up a different name for it from main router. 

1) Verify Password on cell phone by forgetting Wi-Fi 
Go to Settings-> WiFi on your phone, Tap (or Long press  the router's WiFi name  ->Forget / Delete the network 
Please reconnect to your router wifi and it will ask for a password, will it accept your router password?
 

2) Reset: press the reset hole for 10 secs with RE powered on until most LEDs flash once 

3) set up again via Tether app or installation guide.