Samsung Note 4 - Network Locked - But which carrier?

I bought one Galaxy Note 4 on Amazon. I used it in Europe with no problems. Few months are passed and now I moved to China for a while. Well, I can't use my phone here because it is Network Locked (asks for a code at the splash screen). Nothing when I bought it let me suppose this phone was locked to some network, nor I can find any information inside the phone (or the box, or some appearing logo) suggesting to which network it is locked. I am aware there are many service to unlock a phone, but I can't even step up the process of unlocking since I have no idea which network my phone is locked to. My questions: 1) how to know which network carrier I'm locked on? I got some information on the *#0011# menu, but nothing useful, also in about of the phone no hints. 2) Are this unlock services reliable? Thanks for your help.