SIM detected and functional but no mobile network

Hello, I bought my phone about a month and a half ago and everything was fine. However, about two weeks ago I suddenly lost network connectivity without having done any updates or anything. I've tried everything (changing the SIM card, looking for an update, etc.). Furthermore, to make matters worse, the phone is only guaranteed on the Asian market and when I tried to make a repair request on the Sony Hong Kong website, the page simply reloads. I tried calling them, but they only respond in English or Chinese.
Thank you for your help.