@Sagarkhaire 

Hello, what is the hardware version of the WR840N? Is it V6?

Are you able to login to the router web GUI successfully? 

If you cannot login to the router on the Tether app, the most possible reason is that you typed in a wrong login password, remember that it's not the wireless password of the router network, but the login password that you created when you set up the router. If you don't remember that, follow the guide below to factory reset the router, then reconfigure it:
https://www.tp-link.com/support/faq/497/

Follow the guide below to troubleshoot further:
https://www.tp-link.com/support/faq/2810/