changed routers; headless Ubuntu server no longer pulling an IP with DHCP

So, here's the backstory. I recently moved in with a new housemate, who had an existing network in place. Rather than change all its settings, I had hoped to migrate my equipment cleanly using DHCP. That's partly what DHCP is for, right? Well, unfortunately, I have a headless Ubuntu server that doesn't seem to want to play nicely with the new setup.

The previous network was managed by a Linksys E2000 device running DD-WRT, and assigned IPs over a 192.168.1.0/24 format.

The new network is managed by an Arris TG852Gv2, one of the Comcast Xfinity internet-and-phone devices, and assigns IPs over 10.0.0.0/24. On the new device, I have most of the internal monitoring options disabled at the moment as well (listed under Parental Control, but I think some of it is more idiot-proofing, like the Managed Devices section which allows you to blacklist arbitrary devices).

The headless device is running Oneiric, with DHCP enabled for eth0. It is connected via a wire to the Arris device, as it was connected similarly to the Linksys device, and the ethernet port on the Arris registers an active connection.

I've been able to rule out:

• the ethernet cable, as my laptop gets an IP and a stable connection with the same cable.

• the DHCP settings on the Arris device, as every other device in the house (wired and wireless) receives IPs properly.

• the DHCP settings on the server, as they are about as generic as it gets. I don't think the /etc/network/interfaces file has been touched since installation, and it has worked without an issue. I also tested it on a spare Linksys device that I have, that assigns IPs in the same scheme as the previous E2000 - worked fine.

• the most basic "lol power cycle it" sorts of bugs, as I've tried bringing both devices up in different orders, to no avail.

Am I missing something here? Thanks in advance for any suggestions.

tl;dr phantom DHCP issues how fix??