maybe flashing an openwrt factory image with hex edited header could help here. when the manufacturer/product id gets corrupted, the bootloader refuses tftp images with them set to correct values. I've recovered mine long ago by hexing 0xffffffff in , let it flash and then sysupgrade -f 'ed to the same openwrt image in original state again so future upgrades run under the correct manufacturer id again. (search my topic from the past)
See: Need Modified Image Archer C7 v2 Product ID Verify Fail Bricked/Bootloop