iPhone 5 always asks for username when connect to Wifi
The problem is that iOS is detecting your network as an Enterprise network that requires username and password. 
The work-around:
Manually enter the network type into iOS. To do this with iOS 7.1:
Settings / Wi-Fi / Choose a network, and select "Other".
Put in the name (SSID) as it is advertised on the previous screen.
Under Security, choose WPA2 or WPA (try both). Do not choose the "Enterprise" network. Enter the password.
My experience is you have to try 3 or 4 times before it works; don't know why, maybe fat fingers.