No service and searching on the status bar

Dear Apple Company,

I have written this email because I have a hard issue to ask you for a favor.

I have used my Iphone 6 for nearly a year. I bought from USA and it was delivered to me in Vietnam. Everything is going well until yesterday, I cannot connect to a cellular network (it means I see No service/Searching on my device).

I have checked with the carrier and change the new Sim already. Restarting or taking out the Sim card has been also applied but this issue has been occurring until now.

I have check my serial number for the activation lock and it has appeared On. I am wondering this might be the reason of my phone's issue.

Please help me to check my account about that problem.

I hope to hear from your response as soon as possible.

Thank you and Best Regards,

Pink Nguyen.