How Can i work offline,and when connected to Internet Should integrate to my real-time online database?

I'm Stuck With Strange Client requirement.I Made Online CRM for him in .Net framework. Now Client wants when his service Engineer Goes to Customer's Site for service, If that time internet is not Available on his tablet for that location (possibly network issue ) how will he Save data to real-time database online? customer signature and all details can be filled only on Customer's Place .

The Solution I come Up with to Make Mobile Application for that module.store it on device and when service Engineer is reachable to Network all data Sync to online DataBase...

But Problem Is I'm Alien to mobile app development. that's Why i'm Finding any other way that i can save data and use it when available to network ?