Check if those files are in new KF8 format. If I'm remembering things correctly that alt "font hack" via reader.pref edit is in effect only for old mobi format and not for KF8/azw3 books. You can check if that is the case by copying one .azw3 book and opening the book.
So if that's the case the only way to use your custom font in both mobi and azw3 books is to install
this font hack and and after install delete "nobrowser" file from linkfonts folder to activate the font override for browser font/font used for KF8/azw3 books. And then reboot your Kindle.