Mac Address behind a router

1

What mac address will always be written behind a router WAN or LAN?
InternetGatewayDevice.WANDevice.1.WANConnectionDevice.1.WANIPConnection.1.MACAddress
or
|InternetGatewayDevice.LANDevice.1.Hosts.Host.1.MACAddress
|InternetGatewayDevice.LANDevice.1.LANHostConfigManagement.MACAddress|

2

What are you wanting to do? Its not clear from your question.

3

I want to use a unique identifier that is written behind the router that i can use.

4

Most likely it will be the WAN MAC address.
InternetGatewayDevice.WANDevice.1.WANConnectionDevice.1.WANIPConnection.1.MACAddress