Hi there!
If no display is showing, it's possible that the screen could have been damaged during shipment or assembly. Or it could be that the converted videos are somehow corrupt. To check that the screen is working - can you try putting the TinyScreen+ board into bootloader mode? When correctly done, the screen will display "TinyScreen+ Bootloader Mode" on the screen.
For bootloader mode, turn off the TV device (switch should be down), and then hold in the top-left button closest to the micro USB port while turning the unit back on. The most important part is keeping the button held in. Make sure the battery is charged and try this a few times to see if you can get anything to appear on the screen.
I also attached some pictures of the bootloader mode steps.
Let me know if that works for you!
Best,
Réna