Hope you managed to solve your problem, OP. You can use TLS encryption to secure the connection on your MQTT broker. I recommend getting more info from here on how to secure your data. This guide was written by the team behind the MQTT broker Mosquitto and is, therefore, reliable and up to date. You can follow their step-by-step guide to configure a professional, secure and performant MQTT broker. I also have several devices in a network and use this encryption system to make sure no one hacks them. Securing data transfer between clients of a network is mandatory.