You have to sync up manually until the most recent log from primary and then you can keep it back in auto recovery. You do not have any other alternative other than following this when its outta sync.

Do switch a log on primary before you start applying logs manually on standby, that way by that time you apply all the logs, it will not generate another log.(I'm assuming here few logs, minimum time for log switch and applying recovery).

And make sure that before keeping it in Auto mode, log_arch_dest_state_n is 'enabled' not 'defer'.