Hey Mario,
I realize, that this topic has been inactive for some time, but given no answer has been posted, please find my full summary on Dynamics 365 Marketing and Fonts here: fordosa90.medium.com/dynamics-365-marketing-fonts-the-complete-guide-f3df6082710f
As for your actual question, am I correct to assume, you have uploaded your font as WebResource into Dynamics itself? So under advanced settings, customize system (or within a solution)? Your link looks like that. For a font to be available, it needs to be publicly available - easiest way to test is checking in a private browser. WebResources of Dynamics (Dataverse) are behind password.
In addition, depending on where do you store the font itself (Azure, Wordpress, Web server, SiteCore, PowerPages, Portals, ...), you might need to set CORS settings too. All this you can check in the above blog post.
Also, your final comment suggests, that you want to use custom font in Email, and not Form or Page. Referring back to above blog, unfortunately most of the modern email clients (Gmail, Outlook, Yahoo) do not allow custom fonts (web fonts) to be rendered in emails. This is not the short coming of Dynamics Marketing, but rather the email clients, and I am afraid you only have work arounds but not proper solutions for that.
