Just to pick nits, but none of 'em modulate or demodulate, they are technically bridges.
Things I'd check for - and unfortunately they need someone there with a know good working laptop and know how to test all of this stuff
I've only seen this on cable modems, but quite often if you plug something directly in you only get service on the one MAC address - even if you unplug it and plug something else in. Different MAC=no service. Reboot the modem to fix that.
Is the modem in router mode or bridge mode? If bridge mode, is there a router somewhere, as well as a DHCP service, etc?
Is there a DHCP server active, and does a fixed address with fixed gateway and fixed DNS server work? Is there more than one DHCP server active, and the testing device is getting an IP with an invalid default route out?
If cable modem is in router mode, is there another router plugged into it, causing double NAT and address issues with the same subnet?
When testing connectivity,