Add new DynDNS service: https://spdyn.de
Update URL sample can be found under https://wiki.securepoint.de/SPDyn/Hostverwenden#Verwendung_mit_Fremdhardware.
Site is German but the page itself should be self-explanatory.
It would however be the best option to provide a full configurable custom dynamic DNS to the customers. It should be that hard to implement a custom URL using predefined variables.
[Deleted User] commented