How can I enter bootloader with OTA, to upgrade with a mobile application such as Nordic nrfConnect/Toolbox.
This board has only dfu button, but according to the guide https://github.com/adafruit/Adafruit_nRF52_Bootloader in order to enter dfu mode I need also to switch frst button which is not present on this board.
I’m afraid we can’t help with Adafruit’s tutorial, but you might have to add a button to your board so that it more closely matches the board in the tutorial. You will probably want to start by comparing the schematics between Adafruit and SparkFun and add buttons to the SparkFun board to the pins they are on the Adafruit board.