https://www.dyndns.com/ Allows you to have a URL such as www.myserver.com which resolves to your dynimically changing IP address - hence Dynamic Domain Name System (DDNS). It does this by you installing an app onto your server which bascially keeps reporting your IP address, some routers also come built in with this function so you don't need any apps running to perform this.
I have been using this service for years with a home server connected via DSL (with Dynamic IP) and it has never failed.
There is also www.no-ip.com which does the same task.