I have opened a bug report regarding this QT IFW issue.

https://bugreports.qt.io/browse/QTIFW-1481

First, go here http://download.qt.io/online/qt5/ to figure out the url to the version of repositories you need. For me it's linux 64 bit, so I'd click down into that path until I got to http://download.qt.io/online/qt5/linux/x64/online_repository/.
Copy this url into memory.
Then, in the repositories tab of the Qt maintenance tool, select "user defined repositories" (see image in the previous post) then to the right of where you have just clicked, paste in the url you just copied. It's difficult to tell there's even a field there, it's almost hidden. When you hit the right spot the url will paste in and you should be good to go.

mike

You need to sign your installer and app executable using a certificate. You can get such cert. from many companies, just Google for it. It costs between 100-400 US dollars, certificate is usually valid for a few years (1-3).

Once you have the certificate, you can sign any exe with:

signtool.exe sign /t http://timestamp.digicert.com /f yourCertFile.pfx app.exe

And then verify the signature with:

signtool.exe verify /pa /v app.exe

The signtool.exe is part of Windows SDK, which you can get from Microsoft, free of charge.

@gregorburger Yes, we had such an issue last week, I hope it will work soon.