Port forwarding not working despite DMZ and bridged modem

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http://serverfault.com – So I'm trying to set up SSH access to a friend's server, and we've learned from his ISP (SureWest) that their modem is bridged. Next, the router (Linksys E2000) was set up for port forwarding. No outside access. We then tried DMZ to the server's IP. No go. Then, we flashed the router with DD-WRT, still convinced it was a router issue. Port forwarding didn't work, and neither did DMZ. At every step, we've had LAN and loopback access (via WAN IP). What else could be the issue? Thanks in advance :) Extra info: Ubuntu Server, Comtrend NexusLink 5631 modem (HowTos)