I used MKR 1300 WAN with the The Things Network in Europe (868 MHz) with the MKRWAN library.
One has to get the device EUI from the MKR 1300 in order to register the device on a TTN application.
The join, with the gateway not far away, is made at spread factor 7 / 125 KHz, but messages (uplinks) are instead made at SF12 (unfortunately).
I have also to find a way to store the session parameters (join data, frame counter, ...) so to use after sleep.
It is bad practice to make a new join every time one has to send a message.