This is common with SPD devices. They often stay in Diag Mode for only a few seconds unless a service tool "grabs" the port. Make sure your software is ready and clicking "Connect" the moment the port appears. 2. Device Manager shows "Unknown Device"
Try a different USB port (preferably USB 2.0) or a different cable. Some modern USB 3.0/3.1 ports have timing issues with older SPD bootloaders. Conclusion enter diag mode spd
How to Enter Diag Mode on Spreadtrum (SPD) Devices: A Complete Guide This is common with SPD devices
Most SPD phones enter a temporary Diag state or "Boot Mode" through a specific button sequence while the phone is powered off. Power off the device completely. Conclusion How to Enter Diag Mode on Spreadtrum
Diagnostic Mode is a special state that allows a computer to communicate directly with the device's modem and internal configuration sectors via a COM port. Unlike "Fastboot" or "Recovery," Diag Mode opens a communication bridge (usually labeled as SPRD U2S Diag in Device Manager) that lets service software read and write sensitive data required for network connectivity and hardware identification. Prerequisites before Starting