@felmue Dear Felix,
Actually, we have done nearly the same thing.
We have connected this module: HopeRF RFM95W to the connectors on the back of the Core2. That works great.
We have no idea why the LoRa module does not work with the Core2. Maybe a SPI conflict somewhere deep in the background, maybe a power issue or maybe, and more likely, we are lacking some understanding.
Anyway, we have decided to use a Proto board, solder the LoRa module on it and place that on the backside of the Core2.
We have to do that for 35 devices, but it is fail save.
Thanks for all the effort
Regards