Go to Settings -> Carrier and select the network you want to use. Please restart the iPhone after selecting the network.
Once you select a network, iPhone will only use that network. If the network is unavailable or KeepGo has no roaming agreement with the network carrier, “No service” will appear on the iPhone screen and you won’t be able to make or receive calls or visual voicemail, or connect to the Internet via cellular network.
Set Network Settings to Automatic when you are back in USA.