Just to mention a small alternative to dyndns that can work for some cases (e.g. me). If you have a web host somewhere with a known IP, then you can run something like this from cron, meaning that REMOTE_HOST/lastlog "always" contains the "current" IP address of your dynamic system. I use this to make sure I can always log in to my home machine (aka the Ardour build system for Linux & Windows & some macOS builds) as long as I can get into ardour.org and cat lastlog.

#!/bin/sh

ip=`curl --silent ipinfo.io/ip`
ssh -i $SUITABLE_SSH_KEY REMOTE_HOST "echo $ip > lastlog"

exit 0