I use a DG834GT, and DMT works fine with port 23. I'm using the unofficial UberGT firmware, but DMT also worked with the default firmware. OldFogy's suggestion about the firewall could be relevant.
To OldFogy: DMT is a program which allows you to view and modify the connection parameters of the router. It's commonly used to tweak the target noise margin to get either greater speed or greater stability.
Eric