Im assuming you've tried airplay screen mirroring from an iPhone (if you have one).
The sad, yet inevitable response to your problem is that platforms do have their support dropped and the old AppleTV is too old now sadly.
AppleTV is a fantastic platform and in my opinion the best... worth the money and might be worth you investing in the upgrade. That said the NOW app on the appleTV is not the best - forget it if you expect 5.1 sound through Boost.
Plan B is to spend a tiny bit on something like a Firestick. The Lite version is only £29 and that's when its full price - you'll get a decent streaming device and more importantly the most up-to-date NOW app on any platform (NOW seems to favour Amazons's devices now they no longer produce their own branded hardware).
Sadly I think you will have to consign the old AppleTV to the recycle bin.