How to find out the MAC address of the WAN port of the router without access to the router?

Question

Computer networks

Good afternoon, I have a working router that is being used. I want to reflash it to DD-WRT. but my access to it is closed (more precisely, I set a password and forgot) and there is a temporary poppy address that is tied to the provider.
I need to know this poppy address. to remember, so as not to lose the Internet after flashing.
The router is in front of the computer, to change the poppy address with the provider, you need to wait for Monday + there is no time on Monday
How can I find out the WAN port.

Answer 1

In Linux, you can set up Internet distribution on Ethernet (in the IPv4 connection settings, select "shared with other computers"), connect with a cable to the WAN port of the router, and then look at the ARP table:
There should be a line with the IP and physical address of the router. Crooked, of course, but this is the first thing that came to mind without the use of specialized software.

Answer 2

cmd -> arp -a 192.168.0.X