Default [SHV-E250L] Can't connect or register to network

Hi everybody,

For a while I was using the SHV-E250L under custom firmware without problem.

Now, I moved to China and I must use my mobile with stock firmware (company policy). However, since I came changed to stock firmware / unroot, the connection to the network is random:
-Somtimes working well
-But most of time i got the messages "impossible to register to the network" or "registration unauthorized"... asking me to call a customer service in Korea. It's like the mobile is using roaming, just because the phone is Korean (but I'm in China with China SIM !)

I don't know if some users of this variant already faced these issues.

I was thinking, if I change the CSC number to China, and reset the firmware via Kies it would be better, but I'm not sure that it's possible.

If anyboby have any idea.

thank you by advance.