How to prevent download from being stopped if network got disconnected?
Well, you need some kind of internet connection to download files. From what you said, the download stops when you disconnect from your network.

If you meant that the download doesn't work over mobile data, check the list of apps with restricted background data access for Download Manager or, of course, Chrome. If one of those apps have restricted background data, disable that option. If it still doesn't work, try a different browser and tell me whether that worked.