App started without internet doesn't gain it when it's available

Steps to reproduce:

1. Turn off Wi-Fi.
2. Start the attached app.
3. `QNetworkReply::UnknownNetworkError` errors start flowing in.
4. Turn Wi-Fi back on.
5. All subsequent requests still fail with `QNetworkReply::UnknownNetworkError`, network connectivity is never regained.

This problem occurs for `QNetworkAccessManager` instances created during network unavailability. It does not occur on Linux.

It's very similar to QTBUG-49760, the only difference is that we disable the network prior to starting the app (compared to doing so while it's running).

Workarounds we found:

• using deprecated `nam.setNetworkAccessible(QNetworkAccessManager::Accessible)` yields `QNetworkReply::HostNotFoundError` errors instead and successes to complete requests once network is regained,
• don't cache `QNetworkAccessManager` instance.