Data roaming should be ON in order to use free data roaming in 140+ countries. You can turn data roaming on or off by dialing #RON# and #ROF# respectively, or via settings on your phone. Roaming in China is 2G speeds. If you need higher speeds or need a lot of data, make sure your phone is SIM unlocked and seek to buy a local SIM for data while in China.
I'm currently in Hong kong with T-Mobile one plus plan, and I’m only able to reach my data speed at max 13kbps not as what T-Mobile web say 256kpbs. This kind of speed can‘t even use google map. So disappointed