Indigo Domotics

Do NOT try to use non-standard ports. That will definitely break things.

According to the Mosquitto docs:

listener port [bind address/host/unix socket path]
Listen for incoming network connection on the specified port. A second optional argument allows the listener to be bound to a specific ip address/hostname. If this variable is used and neither the global bind_address nor port options are used then the default listener will not be started.

So it's only listening on the loopback port (on the rPi). You need to either remove that IP address, or change it to the rPi's actual IP address (that the plugin can reach. Or maybe add another line with the same port and the real IP address. Read the Mosquitto docs.

got it. I'll do a little more digging. Thank you for your patience.

Update: I'm up and running.

Sorry for the confusion before... I was toggling between my lack of knowledge and a new MQTT broker that won't let you adjust the configuration file.

Long story short... I didn't have to change anything for it to work.
For some reason, the MQTT Connector: MQTT Broker device didn't want to accept a change to the device settings. I thought that might be the case, but even after a few plugin reboots and a full indigo reboot, it still wan't connecting when using the same settings that I used from MQTT Explorer.

I decided to start from scrath and created a new MQTT Connector:Broker device with all the same settings and it worked right away. As far as I can tell, both devices are configured the same, but one works, one doesn't.