Could you try performing firmware update via SSH? First you would need to upload the firmware to the TRB device via scp/WinSCP, as instructed here to /tmp directory within the router.
Then you would need to login to the router over a terminal or some SSH client depending on your operating system, or use CLI in the router's WebUI. For instructions, please refer here.
Next, execute the command:
- sysup -V /tmp/TRB1451_R_00.02.03.2_LEGACY.bin
Please post the output, if the procedure fails.
Also, execute another command and post the result: