There are some limitations to what can be pinned to a dashboard.
Make sure that the merged tile is configured correctly. Verify this by running the merged query in data explorer returns the expected results. If successful, and pinning to the dashboard still doesn't work, try creating a new tile on the dashboard, using that same query.
If you're still unable to pin the merged tile to the dashboard, it's possible that the output of the merged tile is not supported for pinning. In this case, you may need to create a custom solution using either the Monitor Workbook or Power BI to display the data in the way you want.