In order to monitor LINE messages on the non-jailbroken iPhone, iCloud back up for LINE should be activated firstly.
Before backing up LINE data, make sure you have:
- LINE version 6.4.0 or later
- iOS 8.1 or later
- iCloud Drive turned ON
Check here for a tutorial video with the steps to backup LINE messages on iPhone.
Or you can also follow the below simple steps:
1. Go to More > Settings > Chats.