After some erratic behavior of the NB and finally a bricked device I looked in the system integration manual and found differences between the use of the POWER_ON and RESET_N pins in the manual and in the library.

I suspect I have bricked one device by using the library restart function too liberally. See arduino-libraries/MKRNB#47 .

I have tried a rewritten variant of the library on a new device, following the SARA-R4 datasheet and systems integration manuals, and it seems to work. I would prefer to remove the use of RESET_N even from the arduino core used.

I have not yet created a pull request, I would like to discuss changes with devs here. See https://github.com/janakelarsson/ArduinoCore-samd/commit/20cfad1d63500e640cd802d8bc88e934305b0ac9