Normally Windows will fail to connect to an IPSec VPN server if either or both the client machine and the server are behind some form of NAT.
However it is still possible to configure a Windows machine to allow such connections via a registry tweak.
I had to look for an alternate VPN system to use when I need to dial back to my home network while on the move to access my media library or when I require a trusted connection or a whitelisted IP.
The next best thing (and least complicated to set up going from PPTP) is IPSec/L2TP, which has built-in support in most current operating systems (including Windows, Linux and Android). Due to its double-encapsulation nature (L2TP performs the tunnelling of data and IPSec provides the encrypted channel), L2TP/IPSec has a more complex setup and configuration procedure, both for the server and the client.
