I have been making a series of tutorials for MQTT protocol. In this tutorial, i have explained how to make an android app using mit app inventor.
source: https://highvoltages.co/iot-internet-of-things/how-to-mqtt/how-to-make-mqtt-android-application-using-mit-app-inventor/
Video tutorial : https://youtu.be/WAimZhU5phs
Github: https://github.com/HighVoltages/Mqtt-in-Android-app---mit-app-inventor