Wrong driver or bad USB connection. You should check in Windows device manager (COM portrs) whether your phone is identified as Preloader USB VCOM port after connecting to USB or not.
moderator friend
Using this tool I've run out of network 4g.
you can contact a umi engineer to solve this problem
Files that are in, ROM are referenced as MTK 6735 and think it should be 6753
Does the VCOM driver have to stay there as long as my phone is connected? When I connect my phone (no battery) it appears for a second then disappears.