A method of operating a network server, for a network comprising a plurality of wireless mobile routers is provided. Each mobile router comprises a wide area network wireless interface to access a cellular network provided by a predetermined cellular service provider and is operable to upload and download data via the cellular network. Each mobile router is operable to monitor the uploaded and downloaded data to generate usage. The method comprises: selectively coupling the server to the plurality of mobile routers via the cellular network; receiving from each of the mobile routers the corresponding usage data; operating the server to access billing data from the service provider for each mobile router; and comparing the usage data for each mobile router to corresponding billing data for each mobile router to determine if there is a billing discrepancy.