Accessing SQL Server from mobile app remotely

Hi, I have created different web services to access my sql server database from a mobile device. The thing is they both the mobile device and my laptop had to be in the same network(wifi). Now, I would like to know what do I need to do in order to using my mobile app from the outside access to a remote database.
Please be specific with your answer, I would like to know if a rest api is the best choice or not, and if it is then what's the procedure to implement remote access and what's the security configuration for sql server. thanks.
I using Xamarin Forms(cross platform) and SQL Server 2017.