Hello to everyone here. When I try to flash factory image it says: "The uploaded image file does not contain a supported format. Make sure that you choose the generic image format for your platform." I'm on OpenWrt 18.06.0-rc2. Thanks for your help.
OpenWrt has started to stores image metadata appended to the actual firmware around 17.01.x (most targets before, some later, some not yet at all) - if you need to flash a firmware without image metadata (old OpenWrt or possibly vendor firmware), you need to force the installation (sysupgrade -F|--force).
Disclaimer: I don't know if the ens202ext vendor firmware is compatible with sysupgrade, check the device's wiki page to be sure.
Engenius factory firmware distribution files tend to be weird. I think it is in two parts. It is hard to get it into a form that can be flashed from OpenWrt.
An OpenWrt "sysupgrade" for a NOR device is usually simply a byte for byte image of the flash starting after the bootloader. If you have another device of the same model still runing factory Engenius you may be able to extract a factory image from it.
Thanks for your replies. I am using openwrt firmware from easywrt.com in other engenius radios and I can go back to factory directly from the web interface. I don't know if they are doing anything special on their software, but until I'm sure, I don't want to force a sysupgrade and risk to brick the radio.