Hi Juergen,
Further to my last message, I found some info on the following link about bootloader mode :–
3DR Radio in Bootloader mode after failed firmware update - #7 by rrr6399
It involves shorting the Ground pin and CTS pin to put radio to bootloader mode. After shorting,upon power up, the red LED goes solid, meaning in bootloader mode. But after clicking the upload firmware button, it still fails…“Cannot put in to bootloader mode”.
The NC pin next to Tx in Crius is CTS pin when referencing Pixhawk pinout.
Any other clue ?