I have an iPhone X running iOS 14.3.1 beta. It has no issue connecting to the Wi-Fi and the app works as expected. Your iPhone 12 should be running iOS 14.2.1. Once you connect it to a Wi-Fi source it should not let go of it unless the source turns off or you redirect it; it has nothing to do with an internet connection.
If an android can connect so can an iPhone. Try deleting and reinstalling the app.