@smounche 

The model you have is not on the Tether compatible list: https://www.tp-link.com/il/tether_compatibility_list/

@Tony I have the same exact problem using Router Archer C50. I using Tether on an iPhone with latest IOS. Just chatted with an online agent and was told to use wifi password for second login and had the same probem. Why do I need to login a second time. How to I get to use this APP?

@Besimp 

When you use the Tether app you need to create a TP-Link ID, once you are log into the app with your ID you then log into the router. If you tap the router and it is asking you for the username and password it would be the default login assuming the router has not been configured (User: admin password: admin). If that does not work, factory default the router, and then try again.

Using the router wireless password is only used when you are trying to connect to the Wi-Fi, nothing related to the Tether app.