Try a different HDMI cable. Just because it works on one TV doesn't mean it will work on all TVs.
Try a different HDMI input.
Connect the box to another TV and try changing the video resolution to match the native resolution of the Telefunken TV.
Try a different HDMI cable. Just because it works on one TV doesn't mean it will work on all TVs.
Try a different HDMI input.
Connect the box to another TV and try changing the video resolution to match the native resolution of the Telefunken TV.