Hi.
I've rolled out a major update today that fixed a bug where some people could not add a server to the app. If you have tried the app before and had this problem, please try again. If you haven't tried the app then why not give it a try?
The app is free with no in-app purchases or ads.
https://play.google.com/store/apps/details?id=com.interactive.croggy.sandstorm_rcon