update: solved - it was the coverage.
If you have been successful with sending data to The Things Network / Arduino Cloud, using an Arduino MKR WAN 1300 or 1310, please share your experience.
skruglewicz and I have tried this. The configuration worked for both of us (Arduino Cloud, TTN, customised sketch upload), but we don't get an active sub-1 GHz connection.
If you were able to exchange data, please share where you tried this, what Arduino/antenna combination, and where you placed your gizmo. And if you want, the sketch you uploaded (remove the Arduino-generated ID from the header comments).
Thank you