I am using the same router, same firmware and use same version of DMT. Set to port 23, uncheck special logon, check Autoconnect on startup.
Ensure you have input correct IP address, login and password.
On occasion it does fail to login, the fix I have found for this is to check special logon, press connect (knowing it will not connect) then uncheck special logon, press connect and it connects straight away.
I rarely use DMT now with the amount of information that can be collected with Routerstats.