Great question! What games are you attempting to play on your device? Have you turned Wi-Fi assist off?
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You can still make calls over the 1xRTT network while connected to wifi.
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Let me ask you from a different way: why do you think you are using cellular data when you are connected to wifi? there is a setting on the 6s you should turn off, goto settings - cellular then scroll all the way to the bottom and turn off wifi assist