Hello @Nirmalendu ,

Welcome to our community.

For your case, here are some suggestions that may help you:

1. Confirm Device Compatibility: Ensure your router supports the Tether app. You can check the list of supported devices here: https://www.tp-link.com/tether/product-list/
2. Network Connection: Make sure your phone is connected to the main Wi-Fi network of your TP-Link router, not the guest network or any other network.
3. Firmware Update: Log in to your router's web management interface and check if there is a firmware update available. 
4. Disable VPN: If you have any VPN applications running on your phone, please turn them off as they may interfere with the Tether app's ability to detect your router.
5. Turn Off Mobile Data: Please disable mobile/cellular data on your phone while using the Tether app to ensure it communicates over Wi-Fi only.
6. Reboot Devices: Restart both your TP-Link router and your phone.
7. Restart the Tether App: Close the Tether app completely (remove it from the background) and then reopen it.
8. For iOS Devices: If you are using an iPhone or iPad, make sure to enable "Local Network Access" for the Tether app (Settings > Tether App > enable Local Network Access). For Android, ensure all necessary app permissions are granted.