How to resync the existing log shipping setup:
1. Take full backup of Primary database in the primary server.
2. Copy to secondary server database backup location drive.
3. Disable all jobs (Backup job in primary server, Copy and Restore jobs in secondary server)
To change secondary datbase into online.
Restore database secondarydatabasename
4. Restore the secondary server database using primary server database backup file with standby /readonly or with norecovery mode.
->Backup job first
->Copy job Next
->Restore job last.
6. View successive schedules for successful job history.
Hope you will re configure the existing non-sync log shipping setup into sync mode.