How do I configure dynamic DNS when my router does not support it?

I have a MacBook which is connected to the net via an ancient DSL router (NAT enabled). I want to **SSH into the server** from elsewhere, but my ISP assigns new IP address every once in a while. I would like to use a dynamic DNS service for "finding" my Mac, but the **router does not support** any dynamic DNS functionality. What (lightweight) option is there to **update** the DNS service with current IP info **anyway**? I remember installing a clunky menu-bar app several years ago, and I would prefer something configurable from the terminal. *NB: I feel it could be a problem when the MacBook is moved to a different network (friends, company) temporarily and attempts to connect to macbookaddress.dynamicdnz.tld suddenly hit their network. Correct?*