Have you tried turning off the cellular data connection manually?
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We want you to be able to fully manage your data usage for your device. Typically, a device will automatically connect to a WiFi source it's been connected to in the past, but if your phone is not doing so, you can check the settings of your phone. To do so, go to Settings > WiFi, then select the information icon(circle with "i" inside) to the right of the WiFi network you're connected to and ensure Auto-Join is on. To decrease the possibility of unwanted data usage, you can also make sure WiFi assist is turned off in the settings of your phone by going to Settings > Cellular > WiFi Assist. However, the best option as Ann154 brought up, would be to disable cellular data while you're at home just in case your WiFi disconnects or the signal becomes unstable.
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