Check your wifi settings and see if it allows the phone to use the cellular data connection if the wifi is weak.
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Thank you for bringing this to our attention. It certainly sounds like you have your Smart Switch turned on. You can check this by following these steps: settings > Wi-Fi > Tap the 3 vertical dots in the top right hand corner > advanced > Tap Smart Network Switch to turn it off. Let us know if this was able to help you.
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