The Foxsat-hdr can’t play video files via the usb port unless you create the required sidecar support files. Three files are required xxxx.ts, xxxx.nts and xxxx.htm.
Raydons AV2HDR will create all three files for you from compatible source content.
You may want to look at the custom firmware for the Foxsat-HDR. This will allow you to play the converted content using your home network (eg stored on a laptop or NAS drive). No need to use usb at all.