You probably need to enable port-forwarding on your router, if you haven't already done so. Keep in mind, this can create a bit of a security issue, so make sure you fully trust the app (I'm not particularly familiar with it, although I have heard of it).
The process to do this varies from router-to-router. Check the documentation that came with it, or if it was supplied by your Internet Service Provider (ISP) ask them about it.