Restoring ZTE mf286r original firmware

Hi

I successfully installed Openwrt on my mf286r following the guide https://openwrt.org/toh/zte/mf286r. I made all the backups as suggested in the guide. Now I would like to revert back to the original firmware, because of the modem instability in Openwrt.
Can anybody provide some suggestions about the commands to return to the original firmware after booting OpenWrt initramfs image via TFTP ?
Thanks