Iphone 13 Max Pro 5G to 4G switching issue

My Iphone 13 Max pro in China is having issue when switching from 5G to 4G. It is happening regulargly. I normally have a call over facetime and i know for sure the it stop during the time of switching. sometime this switching can cause no network signal and it can last between 1 min to 5 min. Can you tell me why it is like this?