Unlocked or Locked iPhone 6s ???......

Hey everyone hope your all having a wonderful day. I was hoping someone can help with a small issue i have, well basically i have an iphone 6s running ios10.1.1 that was connected to metro pcs back in the states. I was able to use my iphone fine with any carrier (sprint,tmobile, metro are the ones ive tried)(dont know how it can get both cdma and gsm but im not tech savy) anyways im currently in mexico for the next few weeks and it seems like my iphone does not support any of their sims. Ive try Telcel,movistar and even ATTnT all gsm networks and no luck.It gives me the famous "sim not supported" message you get when a phone is locked to a carrier. I can go to my settings, scan and choose a local network provider like TELCEL. When i do choose the service provider it confirms the one i selected with a blue check mark, but as soonest i insert the telcel sim card it restarts and takes me back to startup where it indicates me the "sim not supported" message, i can see the signal bars and LTE but i cant bypass this. My questions is this, can the phone be unlocked only for US carriers? ive tried everything from network resets and try restarting with my computer and no success, i really hope someone can help me... thanks again