How can server PC access the SQL Server data of Clients PC u

I have  more than 100 clients on different networks (not on LAN) e.g. one is in USA and other in UK and other in Dubai and so on.
My all clients PC using the same Windows application and their own local DB.and using internet with Router.as you know due to router public IP always change after 24 hour. 

My server PC is in Australia and server is using web application.


Client PC can get access server PC using Windows Services.
But my question: how  server  pc will get the data from client PC's using web services or any in your mind?
Please give any good Idea with example of sample code, please don't give me any huge theoretical link to read. I want just sample code which I will write in my server side Web application and it will fetch data from clients PC .
Thank you.