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.