I wrote to the support a few days ago if it is possible to run FCC on 510. I got an answer that it could be reduced by dji assistant on 0400 so I bought it. And it turns out that 4 days ago they blocked this option. I bought a license which is currently useless in mavic 2.
Sorry guys, right now theirs nothing we can do. DJI must have found a bug on the older firmwares and pulled them to stop people downgrading. )510 is booby-trapped and blocks downgrades
