Solution:
1. Ensure you are entering the correct Host URL into the device. URL must match in regards to HTTP or HTTPS.
2. Are you on FOS Version 3.11.3 or higher?
In 3.11.3 this version of Mobile requires .NET 4.0 which is likely already installed on the server, however a change may need to be made to the IIS Manager.
In the IIS, you will need to create an application pool that mirrors the current one with the only change being it uses .NET 4. Then select the FOS mobile web application (default name fosmobilews) and choose basic settings under actions. Click select and choose the new application pool.
1. Open the Internet Information Service Manager.
2. Click + to open the tree view of the server.
3. Select Application Pool.
4. In the actions section to the right, select Add Application Pool.
5. Select the drop down box and select .Net Framework v4, name the application pool, and click "OK".
6. Select FosMobileWS
7. In the actions section to the right, select Basic Settings.
8. In the Application Pool section select the newly created application pool and click "OK".