Official comment
Hello Michal,
Sorry for the inconvenience.
Will you please first install driver from the link below and check if rekordbox dj recognizes DDJ-RX as well as DDJ-RX properly controls rekordbox dj?
https://www.pioneerdj.com/en/support/software/ddj-rx/

After successfully installing the driver, please update the firmware in the same page as you did for the first time.
If you still have any issues, please let us know.
Thanks,
CRM, Pioneer DJ