how do I get the dynamic WAN (PPP/SLIP) Interface number in a powesrhell variable

I am a newb to Powershell scripting so I can take no credit for the code here as it has all been borrowed from what I have found online. 

I would like to use a powershell script to add a route bound to a L2TP VPN connection.

First I make my L2TP connection.

Then I use (Get-WmiObject -Class Win32_NetworkAdapterConfiguration) to find the index of the L2TP VPN.

(partial output)

DHCPEnabled      : False

IPAddress        : {10.10.1.5}

DefaultIPGateway : {10.10.1.5}

DNSDomain        : 

ServiceName      : NdisWan

Description      : WAN (PPP/SLIP) Interface

Index            : 589827

Then:

$ping = New-Object System.Net.NetworkInformation.Ping

$ips = $($ping.Send(VPN Provider.com).Address).IPAddressToString

route add $ips mask 255.255.255.255 192.168.1.1 if 589827

This fails:

route.exe : The route addition failed: Either the interface index is wrong or the gateway does not lie on the same network as the interface. Check the IP Address Table for the machine.

When I run Route print I see in the Interface List:

0x130002 ...00 53 45 00 00 00 ...... WAN (PPP/SLIP) Interface

This Works:

route add $ips mask 255.255.255.255 192.168.1.1 if 0x130002

But the problem of using 0x130002 is that it changes each time the L2TP VPN is connected. 

So, how do I get the dynamic WAN (PPP/SLIP) Interface number in a powesrhell variable?