iOS notifications with public images on a private server

I’m wanting to keep my home assistant server running privately on my local network and use camera-based notifications using the iOS mobile app. When I’m home (phone connected to wifi), the mobile notifications work great. However, when I’m away from home (I don’t want to expose my hass server to the internet at all), I’m wanting to upload camera images someplace online and send mobile notifications that reference those public image URLs. Is this possible?

The documentation seems to suggest that it is possible. When testing the following snippet (from the docs) I do get the mobile notification, however, I don’t see a thumbnail, and when I expand it, it says, “Failed to load attachment - URLSessionTask failed with error: The internet connection appears to be offline”. What am I missing?

automation:
  - alias: Notify Mobile app attachment
    trigger:
      ...
    action:
      - action: notify.mobile_app_<your_device_id_here>
        data:
          message: "Something happened at home!"
          data:
            image: "https://www.home-assistant.io/images/default-social.png"

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