Recently, a variety of wireless access points (APs) have been deployed by many organizations to meet different traffic demands. Previously, we proposed an active AP configuration algorithm that activates or deactivates heterogeneous AP devices depending on the traffic demand and network topology, in order to realize the elastic Wireless Local-Area Network (WLAN) system. Unfortunately, in this algorithm, the locations of the mobile routers have been fixed as the algorithm input, although they can be placed flexibly as portable APs using cellular networks. In this paper, we consider the elastic WLAN system with mobile routers which can be placed flexibly. The dynamic mobile router placement is proposed for the active AP configuration algorithm to meet stringent traffic requirements. Specifically, if some hosts fail to meet the throughput requirements, the locations of additional mobile routers are dynamically determined by applying the K-means algorithm to maximize the number of those failed hosts in its coverage. The effectiveness of this location determination is demonstrated by numerical examples with three network scenarios.