Always transcode affects only supported devices. When an unknown device is encountered (like your STB) transcoding is disabled because it is not known to which format it should be done.
What kind of STB do you have? There are two ways to fix this, add some profile for your STB so it becomes known, you need to be tech-savvy for this. If you are up to it we can provide additional guidance. Otherwise you can associate the IP of your STB with some existing device profile and as long as this device profile is similar enough to the capabilities of your STB you should be fine.