UMobile & Xiaomi devices seems not matched untill this days 🤦🏽‍♂️ It has something to do with the U Mobile APN setting. Should you set to UMobile 4g if I'm not mistaken. Somebody has posted the solution last year but I can't remember well where I read them. Either in Xiaomi devices subsubforum in Android subforum or Lazada Discussion thread in Kopitiam section.