Xxxxx.duckdns.org refuses to connect (shared grafana url in docker synology)

I did a fresh install of grafana. Imported a dashboard and everything is up and running without issues.
The link of the shared dashboard is exact the same as in the previous version of grafana.

when I use the url in a browser the dashboard is shown, however when I use it in an I-frame connection is refused.
Must be something I do have to configure but cannot figure out what it is.

Any suggestions ?