Unique Reach metric in Looker studio

The reach metric is a unique metric. Unfortunately, unique metrics are not able to be summed up in the dash boarding applications such as looker studio in this case. In case, the Reach metric is really important, then we might have to create an additional flow in the app that will extract the data monthly (if you require monthly reports), and then the reach metric will be accurate.

This following article exactly explains the issue in this case.