Open the device manager in your Windows, and look for a new COM port when you connect your powered off phone to USB. If you don't start flashing your phone, the new COM port (preloader usb com port) will appear for a few seconds only.
Never more a Umigidi Cellphone. There are no updates for Android 7,8,9. Only xiaomi Cellphones for me. There are better support, better Android, better Custom ROMs on XDA.