You tried to flash/root the Note 4X and failed. As a result, the phone becomes dead boot and can't turn on at all. Trying to enter Fastboot and EDL Mode using Deep Flash Cable did not work.
When the firmware update process Xiaomi Redmi 3S to be completed, the error message dummy bin not found appears in XiaoMi Flash Tool. Based on experience, the issue occurs if you use Fasboot ROM MIUI 8 Global Stable v126.96.36.199. Follow the step by step below to fix the dummy.bin not found problem.
Xiaomi officially locked the bootloader by reasoned to improve security system. Of course not all devices, only the latest. The impact, we can no longer flash device through fastboot. Another method to flash Xiaomi phones is through EDL Mode. But there are still many Xiaomi users who do not know how to get a device into EDL Mode, the reason is enter EDL Mode is more complicated than going to Fastboot. Sometimes special tools are needed.
One of the benefits of unlocking the bootloader is you can install Custom ROM or TWRP recovery via Fastboot Mode. There are two ways to unlock the bootloader, officially and unofficially. Officially, you must make an unlock request to Xiaomi Developer Team. Unofficially, we need a custom file like emmc_appsboot.mbn but be careful, changing the emmc_appsboot.mbn file can cause your device to be brick.