To resolve the issue please follow the troubleshooting steps provided in the given links:For Battery Drainage:http://www.samsung.com/in/support/skp/faq/1101689
For Network Issue:
1) Ensure that network frequency of your device is selected on Auto-mode.
Settings > Connections >More Networks > Mobile Networks > Network Mode > Select Auto mode (GSM/WCDMA/LTE)
2) Search for the Network Manually by the steps as mentioned:
Settings > Connections > More Networks > Mobile Networks > Network Operators > Select Manual Search.
3) You may check by exchanging the SIM in SIM slots and also try using different network operators.
If still issue persists then we would suggest you to contact your service provider.
Team Samsung Members