If you see the error "Content not supported" on your Fire TV after you stream to it from the mobile FITE apps please restart the Fire TV. Then use it as a screen again for the show you want to watch but do not change the show.
This error happens with the second show that you stream and each change of show requires a restart of your Fire TV. We are working on fixing it in our next version and apologize for the inconvenience.
Alternatively you can search for the dedicated Fire TV app on the Amazon appstore or download it from here: https://www.amazon.com/Flipps-Media-Inc-FITE-Wrestling/dp/B07GHNNRTL. Sign in with your FITE account and watch!