The question to ask is, why do you want access to internal Qt Company repos?
The harmony website seems to point to Qt 5.12.12 which you can't even get anymore via the maintenance tool of Qt, the last one available for me there of the 5.12 series is 5.12.11. support for 5.12 was dropped in 2021 and it was also the time of the open source/commercial restructuring. So everything is murky anyway :(
the archive of qt5.12 you can still find on https://download.qt.io/archive/qt/5.12/
not sure of that includes the sources & precompiled libraries, only the sources or only the libraries