OpenEnergyMonitor emonTx Arduino shield to MQTT bridge via a D1 mini. The D1 is using the RFM69Pi module, which provides a serial port output of the received radio data.
https://wiki.openenergymonitor.org/index.php/EmonTx_Arduino_Shield
https://wiki.openenergymonitor.org/index.php/RFM69Pi_V3
WiFiManager for automatic WiFi configuration