Workflow with two email reminders

Here is scenario I'm trying to solve, but I have failed so far. I'm using SharePoint Online form Office 365. with SharePoint Designer 2013.

First day every month users in security group shout get notification to submit reports(docx or xlsx). If the report is not submitted within 7 days form start date there should be email reminder sent out to this same security group. If the report is not submitted within 14 days form start date there should be email reminder sent out to this two security groups.

Reports might not be submitted to SP site but on a local server so there must be an option for users to mark the files as submitted.

I'm workflow beginner so I'm looking for full solution.

Thanks in advance. Schaboo