Have you run the font detection tool? It’s under “Tools → Fonts → Look for more fonts”.
If you can run TeXmacs from the terminal it should tell you which font files it is picking up while doing the font detection. I have STIX 2 installed and it is being picked up by TeXmacs.
If it’s not being picked up, it may not be installed in a directory TeXmacs is looking in. For Windows these should be
I don’t have a Windows installation at my disposal, so I can’t say what these path are exactly. You can find out the value of environment variables by inserting a Scheme session into a TeXmacs document and evaluating one of these: