iOS Wi-Fi Assist
Wi-Fi Assist will automatically switch to cellular data when you have a poor Wi-Fi connection. It was designed to enhance your mobile browsing experience.
Why is this important to check?
Wi-Fi Assist is on by default on all iOS devices running iOS 9+. When Wi-Fi Assist is activated, you'll see the cellular data icon in the status bar on your device. If this occurs while connecting your controller to Wi-Fi, the phone may accidentally attempt to connect your controller to your mobile data plan instead of your Wi-Fi network; if this happens, the Wi-Fi pairing process will fail. We recommend putting your phone into airplane mode or disabling Wi-Fi assist while connecting the controller to Wi-Fi to avoid any possible interference.
How do I turn off?
To disable Wi-Fi Assist, go to Settings > Cellular. Then scroll down and tap Wi-Fi Assist. For screenshots, please see this Apple support article.