Problems connecting to VPN on iOS 8 w/ OS X Server VPN
Setting up VPN isn't necessarily as straight forward as one would hope. The first issue is how to pass the connection from your router to the OS X Server. That is done via port forwarding in the router to your server's static (or reserved DHCP) IP address inside the network.

To make the connection work, you need to know your WAN IP address. If you're using dynamic IP addressing with your internet service provider, you need to have some means of updating DNS any time your IP address changes. DynDNS is such a service (there are others).

Once you've got DNS and port forwarding happening, then you'll be able to determine whether OS X Server and or or iDevices are configured correctly by carefully noting any successful connections vs. authentication failures.

Google/YouTube have a large number of videos regarding the configuration of VPN on OS X Server. They may prove useful to you.