MQTT TLS protocol

There is a tutorial about cirrus, it mentions the extra exercise, that replace Non-TLS MQTT port 1883 with TLS MQTT port 8883, as below

At this point you have a fully functional system that can be expanded or modified as required. Below are some additional activities you may want to try on your own.

  • TLS enable the MQTT Distributor module and disable port 1883
    • Note production MQTT Infrastructures should never run on the open Internet without using TLS enabled MQTT connections!

after I tried, and adjust the setting on transmission tcp://, the connection is still off, can someone please advise how to solve it? thanks