Issue is solved now! I connected MISO, MOSI, SCK and RST to the pins with the same names of the ICSP connector and changed the pinmap to:
const lmic_pinmap lmic_pins = {
.nss = 6, // chip select on SAMD21 mini
.rxtx = LMIC_UNUSED_PIN,
.rst = LMIC_UNUSED_PIN,
.dio = {2, 3, 4}, //DIO0, DIO1 and DIO2 connected
};
Thanks to you all, for the tips given 
