Resume log shipping from a specific transaction
I have manually reset the log shipping by updating the last_restored_file column in the log_shipping_monitor_secondary table on the standby server to the last transaction log that was successfully restored.

Something like this:

UPDATE lsms 
SET lsms.last_restored_file = '{transaction log file}'
FROM dbo.log_shipping_monitor_secondary lsms
WHERE lsms.secondary_database = '{DB Name}';