sound frustrating... a couple of bonehead things which you probably checked already, but... did you check to make sure the DVR is set to DHCP?
Also make sure that the PC you used to check the DVR doesn't have like Mcafee blocking the port, I know I nearly pulled out all my hair trying to get a damn PC to find my Avtech 760, and then realised that Mcafee blocked the port on their stupid firewall. Then again, you probably wouldn't be able to telnet into the thing...
Try starting from scratch, depower the DVR, reset the network settings, reset the router settings, and try again.
Good luck.