UK/US Applications Snags

Does anyone know how to download US Apps (i.e. CitiMobile) from an iPhone 5S that I bought in the UK?

I bought an unlocked iPhone 5S (supplied by Apple) in the UK and have moved to the US. I now have a US T-Mobile SIM Card and account, but cannot see or download the US versions of applications, such as 'Citi Mobile US'. Having spoken to Technical Assistance at Citibank, I am informed that there is specific UK software/hardware that prevents me from seeing these applications in the iTunes Application Store...? Is this right? Can I fix it or am I perpetually tied to a web-based solution?