Hi @Namshid,

If you cannot configure the TP-Link range extender successfully, please refer to the situations below to troubleshoot.
Case 1: The extender's default Wi-Fi network (TP-Link_Extender_XXXX) doesn't appear.
    a. Press the RESET button (hole) with a pin for 1 second (all lights on the extender would flash) to return the extender to factory default.
    b. Put your device next to the extender.
Case 2: Can't find the host router's Wi-Fi during setup.
    a. Put the Range Extender close to the host router.
    b. Ensure the host router's Wi-Fi is not hidden. If it's hidden, you may manually input the Wi-Fi SSID/password for the Extender.
    c. Change the router's 2.4G Wi-Fi channel to 1/6/11 and 5G to 36/40/44/48. You may refer to this link or contact router support to change:Changing Channel and Channel Width on a TP-Link Router (new logo) 
    d. Reboot both the host router and the extender, and try again.