It puts you to the other partition, which may still be the OEM firmware but may also contain LEDE, if you have flashed LEDE or Openwrt several times.
Both OEM, Openwrt and LEDE do flash in a round-robin fashion, so that the current partition is kept intact (as an fallback option) and the flash is performed for the other partition, to which the boot is then switched. So, after several flashes, neither partition may contain the OEM firmware.
You need to use the sysupgrade command from SSH console (using putty or something) and use the “-F” force flag to override the image format check.
You can download the OEM firmware with wget to /tmp and then flash it with sysupgrade. Adjust the commands below to the real link and filename.
commands for you:
sysupgrade -F -n /tmp/oem-firmware-filename