Receive LoRaWAN uplink messages from The Things Stack (TTN) via its MQTT server

Steps

1. In The Things Stack console, generate API credentials for your application: create an API key with the traffic:read permission under Applications > your-app > API keys; use this key as the MQTT password
2. Connect your MQTT client to the regional cluster endpoint (e.g., eu1.cloud.thethings.network) on port 8883 with TLS; set Username to <application-id>@<tenant-id> (for The Things Network the tenant is 'ttn') and Password to the API key
3. Subscribe to the uplink topic v3/<application-id>@<tenant-id>/devices/+/up to receive uplinks from all devices; the + wildcard covers all device IDs within the application
4. Parse the JSON uplink message: the decoded payload is in uplink_message.decoded_payload if a payload formatter is configured; raw bytes are in uplink_message.frm_payload as a base64-encoded string
5. To send a downlink, publish a JSON message to v3/<app-id>@<tenant-id>/devices/<device-id>/down/push with {"downlinks":[{"f_port":1,"frm_payload":"<base64>","priority":"NORMAL"}]}; use /down/replace to overwrite the downlink queue
6. Monitor gateway metadata in the uplink JSON (rx_metadata array) for RSSI, SNR, and gateway IDs; use this to diagnose coverage issues and identify devices with weak signal margins