An open WiFi is a WiFi network that is not password protected.
Hence, you should be able to connect to the WiFi simply by clicking on it either via Instabridge or your native WiFi manager. Sometimes the open WiFi turns out to be a captive portal, and in that case, it will look like you are connected to the internet, when you are in fact only connected to the router of the WiFi, without being able to surf.
The best way to check if the WiFi you are on is a captive portal is to turn off your mobile data, connect to the WiFi and open your browser and see if you can surf. If you can surf it is a normal open WiFi, if you cannot surf and are instead directed to a web page asking you to log in, it is a captive portal.
In your native WiFi manager open WiFi with or without captive portals, will be listed as a WiFi symbol without a locker.