Does your phone indicate that you are connected to a Chinese phone network?

No.

I tried to search the network manually but it still couldn’t connect.

Could you double-check your selfserve and addon of your plan to see you had the international voice roaming?  

As well, I recommend you reset network settings of the phone to see if it can help.

The international voice roaming is indeed in my add-ons.

I tried to reset the network settings on my phone but I still failed.

Are you currently on airplane mode? Have you turned it off?

I just tried to connect to the server manually and finally I got the connection.

Thanks for your help.