The URL you're looking for is documented at https://odkcentral.docs.apiary.io/#reference/forms-and-submissions/'-attachments/downloading-an-attachment:
<your-server>/v1/projects/<projectId>/forms/<xmlFormId>/submissions/<instanceId>/attachments/<filename>
The server, project ID and form ID should all be the same if you're using a single form. The instance ID and filename will come from each row in your dataset. Hopefully this gives you enough info to get started! Consider sharing a quick screenshot or description of your dashboard when you have it up and running -- there's always a lot of interest in how people make sense of their media attachments in analysis.