I had a iphone 6s with model A1633, Could I use it in china, like UNICOM or TELECOM.. Thanks

My friend is in China right now with an iPhone 6s A1633 unlocked from AT&T. She was able to get China Mobile and China Unicom activated on the phone. The city she is in does not have LTE coverage, however she does get 4G service and was able to get 20 Mbps download with Ookla Speed test.

If the phone is unlocked and used with a supported GSM carrier, it should work. You may not get LTE speeds for data, but cellular service should work.