Hi everyone,
Please note that the hosted Mattermost mobile apps on iTunes and Google Play are for Mattermost 3.0 and higher., and support multi-team accounts.
If you're still on a Mattermost 2.x server without multi-team account support, you would need to either upgrade your server to 3.0 or compile the 2.x mobile apps from their open source repos.
The "The address does not appear to be a Mattermost server" sounds like an error message with using the 3.0 mobile apps with the wrong version server.
The Android error message is not as clear as the iOS one (which talks about incompatible versions), and here's a PR to fix that for Android..