Mobile WiFi hotspots have become increasingly popular as a new innovative wireless Internet access technology. Although it has received little attention, adaptive, dynamic (re)assignment of channels in a mobile hotspot router — typically, a smartphone or a laptop or a tablet equipped with heterogeneous network interfaces like 3G/4G, WiFi, Bluetooth or ZigBee — is key to mobile hotspot ... [Show full abstract] performance. To fill this important gap, we present a novel scheme, called Ex2R, that finds the best WiFi service-channel by harnessing secondary low-power ZigBee radios accompanied in mobile hotspot systems, enabling intelligent and seamless dynamic channel reconfiguration. Ex2R exploits the RSSI values and clear channel assessment (CCA) outputs from the ZigBee radio sampling interface to measure idle-time fractions on candidate WiFi channels, and determines the channel that can provide the highest hotspot capacity. Ex2R thus provides a mobile hotspot router accurate information necessary for the selection of the best channel. We have implemented and evaluated Ex2R, demonstrating its effectiveness; Ex2R accurately ranks WiFi channels for a mobile hotspot router to dynamically select and switch to the best channel available, thus improving hotspot performance significantly.