Try this way :
upgrade vendor firmware to latest version.
- Hold the reset button and power on the device. wait for about 10s before releasing the reset button.
- upload sysupgrade.bin via u-boot recovery page on http://192.168.1.1
You're probably correct, but the git commit says flash initramfs through the webui, then sysupgrade.
the latest vendor firmware is probably checking the file extension ?
looks like the extension is also .bin, so i don't know...