Due to the not exactly advantageous layout and management of the SPI Flash, the best way to use the device is to replace everything rather than trying to stay compatible with the vendor firmware. If you just want to try OpenWrt and still plan to go back to the vendor firmware, use the non-UBI version of the firmware which can be flashed using the vendor's web interface. In this case, download the non-UBI variant of the device firmware openwrt-mediatek-mt7622-linksys_e8450-squashfs-sysupgrade.bin and upload it in the vendor firmware Web UI upgrade page.
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