Applying Transaction Logs to an existing Database
You have to start over and restore full and diffs with no recovery and then the tlogs  with stand by option. This will allow you to restore tlogs later as well.

You can configure logshipping with option of stand by, so you can read from the secondary database.

There will be a .tuf (transaction undo file) file created when you restore tlogs with standby option.

For manually restoring tlogs use restore gene or you can just use logshipping for automated restores.