Hello   @Rahulchanana 

If range extender set up failed or failed to connect to router wifi, here is a troubleshooting guide that should help

Has connected to TP-Link extender but no internet, what can I do?

It would be a great step to  verify router Wi-Fi password before reconfuire RE. 

•  Forget router Wi-Fi on your mobile phone and manually input Wi-Fi password to verify router’s Wi-Fi password is correct.   ( Phone Settings > Wi-Fi >tap router's Wi-Fi SSID to forget network or Long press wifi name to delete the Wi-Fi profile).