Actually, when you press the RESET button twice (very short period) - the Arduino Bootloader should be activated.
Indication Bootloader is running:
you should see a FADING (!) flashing green LED. When you see it: the "dft-util" should now connect (or the COM port in IDE to flash your project).
But I have other cases where the Arduino Bootloader is NOT activated:
a) I have to try pressing RESET several times quickly until I get a FADING green LED.
It can happen I need N-times until I get it or even never getting the FADING LED at the end.
b) I press the RESET button, twice, and even N-times very quickly:
I get just a STEADY green LED (not a fading one). And "dft-util" does not work.
SOLUTION: power cycle the board very quickly: unplug USB-C and plug it in again (do not wait
too long without power on board).
Now, the green LED is fading (and "dft-util" works).
Remark:
The Arduino Bootloader uses RTC Backup registers (RTC_BKPxxR). The first one (RTC_BKP0R) - at least - is used by the Bootloader.
So, do NOT touch RTC Backup Registers, e.g. RTC->BKP0R - it will interfere with the Bootloader!
"Try all your best until you get a FADING green LED ...".
If not: you need the breakout board with external debugger.
And cross your fingers you never get an orange LED (from the PMIC, the battery charger):
If so: nothing will work, even the Bootloader will not work, the debugger will not work: you do not have power on the board and the board is "bricked" (but it can be recovered with special "tricks").
ORANGE LED on : very bad: "board is dead" (no power for all components on board)
STEADY GREEEN LED : power cycle the board (quickly) - get a green FADING LED afterwards (OK)
never a FADING green LED : Bootloader not available ("dft-util" will not work)