The iOS 11 has been configured to disconnect from wifi when the iPhone goes into sleep mode.
Follow the steps below to set up an iPhone to be "whole home" network friendly:
- Go to Settings, Cellular, and scroll down (all the way to the bottom) and find the Wi-Fi Assist option and turn it OFF. When ON (factory default), this option immediately connects your phone to the cellular network when the iPhone believes the cell signal is better than the wifi signal. Turning Wi-Fi Assist OFF gives the iPhone time to find the best (on next) milo AP without constantly switching to LTE (which often causes network connectivity issues) while you are walking around your home.
- Go to Settings, Wi-Fi, and connect to the network of your choice. Press the i button to the far right of the network name and turn Auto-Join ON. When Auto-Join is OFF (factory default), you have to manually reconnect every time you unlock the phone.
The combination of these two settings makes for a much more friendly iPhone "whole home" surfing experience.