I would suggest doing a cloud backup, setup your Nokia account and then do the full factory reset. Start fresh. Or take help from Nokia Lumia 830 Manual and get complete information about what lumia experts suggects to fix this issue.
Users of Nokia Lumia 1520 can either install the latest update through the Push Notification or go to Settings -> Phone update -> Check for Update to manually download it. Then go to Settings -> Mobile + SIM to enable 4G.