Launching MQTT broker

Unable to get an MQTT broker launched onto fly. Tried following the appkata example from 2020 but it doesn’t seem to work any longer, and I’m not sure what needs to be done to fix it. Following the guide to a tee works until testing stages, where I run mosquitto_sub -L mqtts://<username>:<password>@server.fly.dev/# --cafile certs/rootCA.pem, and get back Error: Protocol error. Anyone successfully deployed a MQTT broker to Fly and wanna share?

Thanks!