Good morning @Roelvdv,
I had the same issue with arduino nano IoT 33 as it is based on SAMD21 MCU I found a solution based on your statements. Thank you so much for your great guidance.
There is the same issue also in another page of forum (LoRa Basics MAC on Arduino 33 Nano - #28 by descartes).
I put a video also next days in my youtube channel it could be helpful:
Make Lorawan node with SAMD microcontroller based boards using LMIC lorawan library - YouTube.
Grateful and best wishes,
Amin TAYEBI