Sounds cool man. I know that it is possible and sounds like it functions pretty well for you, but because this is cellular HTTP will use more data than MQTT and cell data is expensive, and regardless I am just interested in giving MQTT a try.
I just found this link which had some great suggestions if you get a minute check it out and let me know what you think. MQTT mosquitto application
But instead of an RPi this guy used a VM so I don't think port forwarding was an issue