PSA to all friends with iPhones who updated to the latest OS - go into your settings, select "Cellular", scroll to the very bottom of the menu and switch off the "Wifi Assist" feature. Apple has sneakily added this to iOS 9. It automatically uses cellular data when wifi connection is poor, even when connected to your network.
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I love that my Apple Fanboy continues to teach me about the products which I so enjoy using!