I am just an amateur but my recent experience demonstrated that the APN often changes to something not useful or goes blank when you boot up in a new country. This happened to me a lot with Toggle in Nov/Dec 2013 and ATT in China last year. What I learned was to research what APNs are working where, put that in a memo on the phone locally, and if data doesn't work, look at the APN first. XDA is a good source for this but you have to drill down through the teenage whining to get to the useful stuff (kinda like the AA forum).