As I understand, your intetion is to use the router as a file server on an environment with 21 clients. The Asus router is not intended to be used as file server. Even when this capability can be exploited it is very limited. I would recommend you to split the functionallity of your network in two: the router to route traffic and a file server to handle the files. If you have a limited budget take a look at file server solutions like qnap, netgear or dlink.