@Abid12 

May I know your TP-Link account? If you have already changed the username and password, please ensure you typed them in correctly.

If possible, try with another phone and see how it goes. BTW, are you login to the C20 when connected to its wireless locally or from an outside network? Is the phone an iPhone or Android phone, what is the OS version?

If still cannot login, please show us the login failed error message, which may give us some clue.

You can also try to power off then on the router, as well as login out the TP-Link account on the phone then login again to confirm.