Well, best thing is to take a step back and look at all the puzzle pieces. First, you don’t need to mess with port 8080 unless you changed the port or are using OSPi (Raspberry Pi). So, your first issue might be the port. Also, router might not support NAT loopback. If that’s the case your port forward will fail when trying to use your outside IP inside your network. Therefore, test the port forward against the cellular network, if possible.
Second, you mention apps/sprinklers, this is related to the *old* web app which is no longer maintained. Furthermore, this requires a separate server to host the files for you. The focus is now on the native apps available in the various app stores (Apple, Amazon, Blackberry, Google, Microsoft, etc).
Save yourself the headache and set the static IP using your router (against the OpenSprinkler’s MAC address). This way the OS is configured for DHCP which makes software upgrades easier.
In summary:
1) Know your outside IP: http://wimi.com
2) Port forward the OpenSprinkler port (default: 80).
3) After installing the native app, use your external IP for the device IP.