How to obtain MAC address of a remote computer through router

Does anyone know how to get the MAC address of a computer?
The computer is in different subnet and connected through a router.
When I used the SendARP() function to get the MAC address, it worked when
running within the same subnet.

The problem is that SendARP() seems to return the MAC address of the router.

I tried both arp and nbtstat commands but they only worked when running
within the same subnet.

Does anyone know how this might work?
Thanks,

_____
Danny