Just make sure the MM and OpenWRT are from the same release dates, they are matched pairs.
They are not all from the same release dates, there are some OpenWRT images that do not have a correspondent in the MM directory
Also, there is a folder called Nicehash in OpenWRT but no MM nicehash folder:
https://download.canaan-creative.com/avalon841/openwrt/For OpenWRT open GUI and go to System > Backup/flash firmware to start things.
So just copy the .img file to the original microsd disk?