Internet Gateway code simply treats all adapters with media different from NdisMedium802_3 as WAN. This is not correct of course, but works for most cases and good enough for the sample. I think your intrface reports media different from 802.3 and this causes described misbehave. NDIS_MEDIUM is defined as following:
typedef enum _NDIS_MEDIUM
{
NdisMedium802_3,
NdisMedium802_5,
NdisMediumFddi,
NdisMediumWan,
NdisMediumLocalTalk,
NdisMediumDix, // defined for convenience, not a real medium
NdisMediumArcnetRaw,
NdisMediumArcnet878_2,
NdisMediumAtm,
NdisMediumWirelessWan,
NdisMediumIrda,
NdisMediumBpc,
NdisMediumCoWan,
NdisMedium1394,
NdisMediumInfiniBand,
#if ((NTDDI_VERSION >= NTDDI_VISTA) || NDIS_SUPPORT_NDIS6)
NdisMediumTunnel,
NdisMediumNative802_11,
NdisMediumLoopback,
#endif // (NTDDI_VERSION >= NTDDI_VISTA)
#if (NTDDI_VERSION >= NTDDI_WIN7)
NdisMediumWiMAX,
NdisMediumIP,
#endif
There are too many things to improve in this sample to build a real NAT aplication suitable for different environments and it would take plenty of time to build a desired list. You’d better look at some solutions alreaddy available on market to see what features can be implemented.