| Error Message | Likely Cause | Solution | |---------------|--------------|----------| | “Firmware file mismatch” | You downloaded a file for the wrong hardware revision (e.g., ZLT P21G vs P21E). | Double-check your router’s model number on the sticker. Download the correct variant. | | “Update failed. Please reboot and try again” | Corrupted download or low memory. | Re-download the firmware from an official source. Reboot the router and close all connected devices before retrying. | | Router freezes at 50% progress bar | The flash memory sector is damaged or the file is incompatible. | Do not power off. Wait 15 minutes. If still frozen, use TFTP recovery mode. | | After update, no internet connection | The update reset your APN (Access Point Name) settings. | Go to and re-enter your carrier’s APN (e.g., internet , www , fast.mts.ru ). | | WiFi SSID changed or disappeared | Default settings were restored. | Restore from your backup configuration file, or manually reconfigure WiFi. |
Since ZLT’s official support can be limited, the best help comes from user communities: zlt p21 firmware update