OK, I think I've found the root of this problem. I currently have a Samsung Galaxy S4 with its original Android 5.0 OS and whatever Font i download from Google Play, online apk, or FlipFont, it will keep saying "Font Not Supported". While Android generally allows any font to be installed and applied, Samsung seems to be implemented a sort of DRM to clamp down on font piracy and allows only select Font providers (Monotype seem to be one) to be installed, but the catch is that most of their fonts are not free. I know, it sounds ridiculous to "pay for some letters" while Samsung allows sharing MP3s straight from Samsung Music app. I can confirm this because the only font I could install and apply was from the Galaxy App after clicking the Download button from the Select Font panel in Settings. You can download Galaxy Apps store apk anytime online. I also debloated my phone before (No Samsung Keyboard or Galaxy Apps in the system) so I'm sure fonts don't depend on Samsung Keyboard. Hope this helps clear things.