Okay, I've got it running and first message was sent and received!
Now, the signal is really weak, it doesn't work anymore if I am more than 300m from the gateway. So I suspect my antenna not to be really working, for now I soldered a 10cm wire to the ANT pin of the RFM95W. Does anyone have suggestions on how to improve this considering the device needs to be as small as possible?
Thanks!
René