TL-WA830RE V2 with Gargoyle: cannot restore to original firmware

I have what seems to be a defective TL-WA830RE V2. It shows "connected" on the status page after following the wizard, but it does not communicate with the router. Tried on 3 different routers (a Huawei, a TP-Link and an Asus) with the same result.
Wanted to confirm that is not a firmware issue, so I installed the latest Gargoyle ( I've setup the unit as a Wireless Bridge/Repeater, no joy. I can see it on the main router as being connected, the status page says it's connected, but the clients that connect to the repeater cannot access the internet (they can't even get an IP).
Now I'm trying to revert back to the original firmware. Tried System->Update Firmware. It shows a page with "Upgrading, Please Wait.", but nothing happens . If I navigate back on the web interface it's the same firmware, the upgrade did not succeed.
I've tried to scp the .bin file into the device, but it looks like sftp is not installed: bash: /usr/libexec/sftp-server: not found
How can I revert the repeater to the original firmware?
Thank you.

You have to use firmware without bootloader - and make normal update by internet browser.

Thank you. That worked.
Now to replace this unit with a (hopefully) working one.
Have a great day.