Need to get MAC, URGENT Help please

I am remotely connected to one of my clients RB1, I want to connect the another RB2 which is connected to the RB1 I am accessing remotely.

Is there a way I can get the MAC address of the port of RB2 that is connected to RB1 so I can MAC Telnet via RB1?

Neighborhood, ARP, etc shows nothing

If you can provoke RB2 to send something, e.g. just reply to ARP request, then you can use Tools->Packet Sniffer to capture the traffic and get MAC address from there. If not, you might use the same and hope that RB2 sends something by itself.

When it is already dead, start a trace on the port of the first router and have the second router rebooted by the client.
You should see some activity when it boots.

Thank you @Sob and @pe1chl for your quick responses.

Unfortunately could not get this working, will contact client Monday and take from there.

You haven’t written anything regarding the physical accessibility of the devices, maybe the customer could read the MAC from the device’s sticker before you jump into the car?
The MAC addresses form up a contiguous block, the sticker normally contains the address of ether1 and wlan1.

Apologies, as @sindy pointed out, I should have included at the time of my OP, the client has locked up and left for the weekend already