computers app on ATV problems

think you have to be more specific

it will only work if computers on the same network have sharing enabled

Use Home Sharing to share media from a computer to other devices - Apple Support

TV app on Mac Mini uses file location on NAS and all media is available and until last week all was also available on the Apple TV Computer app. However, that is no longer the case, now only the media in the Mac Mini Movies Home folder is available on the Apple TV Computer app, despite the media located on the NAS still being available in the Mac Mini TV app.

What needs to be done to make the same media on the NAS that is available on the Mac Mini available on my Apple TVs?

if it was me then I guess I would try to remove it from the mac mini lib and then add it once more

personally I generally use the synology move app and or the vlc on my appletv to directly access the NAS rather than having a middleman being the mac mini, the more links to the chain the more risk of a weaker link