Step 1: Load Fastboot Mode via Sigma Box Connect the dead STK-L21MDV to your PC using a USB cable. Open Sigma Software. Use the "Load Fastboot" feature to boot the phone into Fastboot Mode. Wait for the device to be detected. Step 2: Flash ROM via IMAGE Download Tool Launch IMAGE Download Software v2.0.0.9. Load the extracted ROM file from the one .zip you downloaded. Select the connected device and begin the flashing process. Wait for the process to complete successfully.