I discovered this thread just now - this looks similar (not identical) to the GS1920 firmware file format I'm currently trying to understand. See if some info from this thread helps: Support for Zyxel GS1920 series (GS1920-24HP)
And you have references to ThreadX and RTL839x, so this is not running Linux but based on an RTL839x platform - again, seems similar to the GS1920 series.