Hm, I tried to check it out (I run my own synapse server) but when I log in I get the error: "Invalid glTF file: Missing asset section." and have no idea how to fix it ^^.
Try updating your Synapse to 1.62 or higher. We added a header to files downloaded from the media repository so they can work on sites using SharedArrayBuffer. Unrelated, but you'll also want to make sure you have a TURN server set up and your media repo upload limit is set to something reasonably high like 100MB.