@BobDar 

If the extender is broadcasting the default Wi-Fi network that has the name TP_Link etc. do a factory reset on it by holding the reset for 15 seconds while it is plugged in. 

If the extender has already been configured and has the partial name of your main network look at the LED lights to see if that indicates it is connected to the router. The wireless password is the same as your main router. If the LED lights are not lit appropriately, factory reset the extender and configure it again as the wireless password provided during installation may have been wrong.