@Umashankar 

Hi, May I know the model number of your range extender?

Please refer to this link to make sure your extender supports the Tether APP first:

https://www.tp-link.com/en/tether/product-list/

Then make sure the smartphone is connected to the default Wi-Fi of the extender.

After that, it is also suggested to refer to the following links:

TP-Link Range Extender Setup (Range Extender Mode)

Unable to Connect to WiFi Extender or Saying Incorrect Password

Thank you very much and best regards.