Hi @David__B, several of us on Discord with a premium account have also raised the same issue in multiple threads. The button still won't show even after performing basic troubleshooting steps, including the ones you mentioned. I looked at Chrome/FF/Edge since I don't use Safari on my PC.
Desktop
Mobile
In the meantime, I noted that the Creative Cloud desktop app would be the way to manage fonts.
Generally, you can upload fonts to both the brands section and the standard font area. Would it be possible to share a screenshot? Also, to confirm, you are logging in with a personal account, not a school or work account, correct?