Hello@smilingmaitri 

Are you trying to log into the modem or your t-mobile account?

For your T-mobile mobile account you should have received an email to create your account or you can click on wachtwoord opvragen at the login page.

For your modem please check your user guide.

In case of your wifi password please pay attention to the capital letters and normal letters, it is also changeable in your T-mobile thuis account.