I guess there are two possible solutions, both of them require to add custom C++ code:
Use the Qt Network C++ components to determine whether internet access is possible.
Or add native code parts to check the connection with Java (for Android) and Objective-C (for iOS) and call them from Qt C++.
The advantage of the first solution is that no platform-specific code is needed, so I would go with that if possible. Using native code might offer more details though, like checking whether WiFi or mobile internet is used for the connection.
See here to learn about Integrating C++ and QML.
This reply was modified 3 years, 2 months ago by GT.