@Nwar 

Regarding the error message, have you checked if you are connected to the guest network? Try to disconnect from the network and reconnect, ensure there are no Parental Control settings for the network or device, then check again if you can login to the router via the Tether app.

BTW, are you trying to login to the router when the phone is connected to the router network, or you are trying to login remotely from an outside network?