trying to use a Japanese sim in on originally USA galaxy s5

I came to Japan after speaking0 to sprint about the possibility of using a Japanese SIM card in my phone. They said there'd be no problems I'll just have to unlock the phone before I left which was a completely doable thing by myself without their help. Now once I set up the Japanese sim I can make calls and text but have no access to LTE on my phone. Whenever I'm outside of Wi-Fi range I get a message saying mobile data connection could not be established LTE: ESM - 30 EMM - X I've called Sprint from Japan and tried to resolve the issue, they have walk me through the same steps three different times assuring me that the phone is unlocked on their end and never with a new result. Is there something that I missing it I need to address before I can connect for mobile data connection in Japan with this USA phone.