Font Management application filters out TrueType Collection (ttc) fonts fonts and hence does not allow viewing or installing them. However, if I install ttc fonts manually by copying them into ~/.local/share/fonts Font Management shows those as installed and allows to view them. Please enable ttc fonts in Font Management application.
I can confirm this issue with Plasma 5.14.5. As a temporary solution, one can: 1. Edit /usr/share/applications/org.kde.kfontview.desktop, comment out NoDisplay=true, then associate font/collection with KFontView in File Associations. Undo the change later if needed. 2. Copy /usr/share/kservices5/ServiceMenus/installfont.desktop to $HOME/.local/share/kservices5/ServiceMenus/installfont.desktop, and then add font/collection to X-KDE-ServiceTypes so that the Install button can show up in the context menu for TTC files.
And possibly copy and edit fontthumbnail.desktop & kfontviewpart.desktop as well for preview and etc...
Just confirming that this is still an issue in 5.18.4. Looking at the code, I see the dialog is filtered with dlg.setMimeTypeFilters(CFontList::fontMimeTypes); Is it just a matter of adjusting CFontList::fontMimeTypes to include application/x-font-ttc?
That's certainly a good hint for further investigations.
It seems not to be an issue anymore with Framework 5.91.0. I managed to install this TTC font (http://fontsvip.com/C-fonts/65295.html) using the "Install from File" dialog.