As i just found out (by reading the OS Manual) you have already built in a function to determine the external ip. When holding B2 and pressing B1 the display shows the correct external ip. So i think it would be easy just to write the gathered value to a specific folder e.g. on a google-Drive or as i mentioned before directly into the OS-Account on opensprinkler.com. It would even be not necessesary to check, wheter this value changes but you could write it every ten minutes or so.