Hello,
You do not need a bootloader image, as you would need to reflash firmware first. Please follow instructions provided here. Since there might be a limit to what firmware version your device can be updated to via bootloader menu, the firmware image to upload can be downloaded from here.
Aside from that, before firmware update, I would like you to login into the device via CLI/SSH (Please find instructions here) and execute the following command:
- hexdump -C /dev/mtdblock1
Make a screenshot and attach it by editing your question.
Best regards,