My iPhone 13 pro max keeps on powering itself off after I call or use it

i bought my iPhone from America a month ago, and I opened the case and tried to use it in Africa-Ethiopia, but it keeps on powering itself off after I call or use it, I have to do a force start (volume up, volume down and side button till the Apple logo appears. But the problem is still there I used all recommendations to a frozen screen by connecting to my pc. Can’t fix the problem, any one can help me out please

Abebe

[Re-Titled by Moderator]