If the phone has a bill, it uses data instead of WiFi. The so-called mobile phone traffic is generally mobile data, you have to open the mobile data switch in the mobile phone, in order to be able to use the traffic of the mobile phone card online.
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WiFi is generally connected to the router emitted, WiFi signals are mostly broadband through the router converted out of the free network, usually connected to WiFi Internet access is not required additional costs.