Download and install the latest of these pods for the current framework version being used:
pahoMqtt (latest for FIN 5.0 and 5.1 is v1.2.1)
pahoMqttExt (latest for FIN 5.0 and 5.1 is v1.0.14)
Once installed, go to the Settings app → Ext → and enable Paho MQTT.