Some people used Serial to Ethernet Adapters to remote access the old phone system according to the contractor who serviced the unit, which are not the most secure! I chose to put an old Pentium II 266Mhz computer at the site with a Null Cable between the phone system and Com1 of the computer running Windows XP Pro and would RDP into this system to edit the phone system that was old but cost too much to replace. I set up a similar, dedicated computer setup to be able to edit a very old phone system that required a modem / serial connection to edit parameters. If you had a computer connected to the digital signage unit that gets updated from another system, thats your best bet.
If you were able to sense on the internet service port that the modem connects to, you will only be able to sense the mac address of that of the modem and no further.
The problem would be that the mac address is non transferrable and the MAC address sensed would only be devices that connect locally to the router, and it is sensed on the switch side of the router and not the service port to the modem.