Hi,
Yes, the plugin workings it not adapted to how you want to use it.
Change the line:
if(!empty($plugin->params['last_cron_update']) && $plugin->params['last_cron_update']+$plugin->params['period']>time()){
to:
if(!empty($plugin->params['last_cron_update']) && $plugin->params['last_cron_update']+900>time()){
in the plugin file plugins/hikashop/order_auto_cancel/order_auto_cancel.php and it will work like you want.
I would love to answer you regarding the configuration of RS Firewall but I've never used it and don't know what the DoS protection does exactly and if they have a whitelist, etc. That's why I recommended to follow the documentation to make sure that your cron is running. If it is conclusive, then no need to change anything in RS Firewall.