Port forwarding - consequences?

I’ve been looking into Cloudmqtt. I’ve got Owntracks publishing to it but I can’t see how…

I have hassio and am using the Mosquitto add-on to control my sonoffs. From what I’ve read I have to bridge Cloudmqtt to my Mosquitto but I can’t find out how to do that.
I’d be grateful for a pointer.

EDIT: I found this [solved] Mqtt over internet? aka: "How to set up cloudMQTT bridge with Hassio mosquitto broker" - #7 by myle but following it gives the following Mosquitto error log - I must be putting the config file in the wrong place?

starting version 3.2.2
Error: Unable to open include_dir '/share/mosquitto'.
Error found at /etc/mosquitto.conf:19.
Error: Unable to open configuration file.