It works now – no idea why. Possibly powering off/on the mDot box did the trick for some reason.
Having the same issue, even with Node Red starter flow. Node red says “MQTT Quickstart connected” but when I go to IBM and enter the device ID, IBM says “waiting for device to send data”.
mqtt-broker is configured like so:
a:quickstart:myApplicationId@quickstart.messaging.internetofthings.ibmcloud.com
:1883
Maybe this is out of date? Wrong port?