Because of JIRA limitation, you can't find the 'Waiting for Approve' issues by using 'Assigned to me' gadget.
You can follow below method to show 'Waiting for Approve' issues on the JIRA dashboard.
From Issues > Search for issues, use JQL as below.
Approver = currentUser() and status = "Waiting for Approve"
Approver is a custom field created in field configuration. All the status including 'Waiting for Approve' can be customized according to actual requirements.
- Save the filter and name it as you like, for example, ' Issues waiting for my approval'.
- Share the filter with everyone or relevant project roles.
- Add gadget from dashboard and select 'Filter result' gadget, choose the filter we just created in step 1.