Fonts not displaying correctly in HTML - Not a MIME issue - fonts not in published folder?

Long title but I'm at a loss here. This is an HTML file being viewed. We have fonts showing up correctly on our internal company computers (we all have our company's custom font installed) but our learners are having a different font appear and (in this case) the letter 'f' is replaced with a box. 

I read through a lot of forum posts and suggested to the backend developers that it's a MIME issue. They let me know that it isn't. All of the Open Sans fonts that are being used are embedded directly and appearing correctly. I was able to find them in the mobile-fonts folder in my published folder. The custom company fonts and Calibri are just embedded into the css. "Per Mozilla developer documentation, we can not safely allow for fonts to be shown when embedded as data elements".

They also said, "Those fonts are embedded meaning they were converted to characters and saved as text in one of the css files. xzThe css needs to refence the files not embed them this should be a setting in the authoring software."

Thanks in advance!