Hi there,
Sounds very cool, You got a Picture of this Apparatus ? How close are they to each other? If you don’t mind…the pair on each end?
HTH
GL 
PJ 
Can you measure RF power ? (radiated) Have you tried without Encryption ?
I notice the OP was using this?;
#define RF_FREQUENCY 868000000 /* Hz */
Seems too close to the Cellular bands?
I’m using these : for example.
LoRa Configuration Parameters
| Parameter | Value | Description |
|---|---|---|
| Frequency | 915.0 MHz |
Standard for US ISM band (can be changed to 868.0 or 433.0 for EU/Asia) |
| Spreading Factor | SF7 |
Faster transmission, shorter range |
| Bandwidth | 125.0 kHz |
Good balance between range and speed |
| Coding Rate | 4/5 (CR = 5) |
More error correction, increases reliability |
| Output Power | 22 dBm |
Max power for SX1262 (can be adjusted via button) |
| Preamble Length | Default (8) | Could be increased for longer-range reliability (optional tweak) |
| Sync Word | Default (0x12) | Used to distinguish private networks (can be changed for custom use) |