fix JOIN WAIT problem with TTN

src/lorawan.cpp

@@ -484,6 +484,14 @@ void lmictask(void *pvParameters) {
 
   os_init();    // initialize lmic run-time environment
   LMIC_reset(); // initialize lmic MAC
+
+// Note that The Things Network uses the non-standard SF9BW125 data rate for RX2
+// in Europe and switches between RX1 and RX2 based on network congestion. Thus,
+// to avoid occasionally join failures, we set datarate to SF9 and bypass the
+// LORAWAN spec-compliant RX2 == SF12 setting
+#if defined(CFG_eu868)
+  LMIC_setDrTxpow(EU868_DR_SF9, KEEP_TXPOW);
+#endif
   LMIC_setLinkCheckMode(0);
 
 // This tells LMIC to make the receive windows bigger, in case your clock is