Your laptop, phone and tablet all remember the WiFi networks they’ve connected to in the past. But their lists of remembered networks can have an unintended — and unwelcome — consequence: When your device sees a familiar network, it may try to connect to it automatically, even though the signal is weak or that network requires you to log in via a ”captive portal” Web page. The end result: You can be knocked offline.
To fix this, you should periodically go through your device’s list of saved networks and delete the risky ones. Here’s how to do it.
Android: Open your WiFi settings, tap the menu button at the top right and tap Saved networks. Then scroll down until you see the offending network; tap it, then tap Forget. Be alert for screwy sorting: On my phone, the list is case-sensitive, meaning networks starting with lowercase letters come after everything else. Also note that the current version of Android is smart enough to ignore saved WiFi networks if they offer no Internet access.
OS X: Open the Keychain Access app (in Applications/Utilities). Type AirPort into the search box (not the more obvious WiFi or wireless) to see a list of your saved networks. Right-click one to delete it. (You can also see them in the Network pane in System Preferences, but that view shows only six at a time and in chronological order.)
iOS: The operating system provides no single view of its saved networks. But if you have iCloud Keychain enabled you can view and edit saved networks from Keychain Access on a Mac, this time by clicking iCloud in the top-left corner before doing that AirPort search.
Windows 10: Open the Settings app, click Network & Internet, then Wi-Fi, then scroll down and click Manage Wi-Fi settings. Scroll down to Manage known networks, select the unwanted one, and click Forget.