Official comment
Sorry, the CDJ-400 was released prior to rekordbox having a performance mode, as such, it did not ship with a license for rekordbox, nor does it qualify as a hardware-unlock device. A subscription for the Core or Creative plan would be required to use it as a controller for rekordbox 6 in performance mode, or if you connect a mixer or other device capable of unlocking the software, the CDJ will function as expected without needing a subscription.