Although this page makes it seem like AppleTV is directly supported....
https://www.synology.com/en-global/dsm/5.0/multimedia

... the thing to keep in mind is that what they're saying they support is AirPlay. There's a difference. Here's the difference:

AirPlay means that if I have the Synology app on my phone, or I'm looking at the Synology library on my PC, then I will get a button (within that app) which lets me stream the video I'm watching to the TV set that the AppleTV is hooked up to. It's just like Chromecast in that regard, in this situation the AppleTV is just working like a Chromecast does. But for that to work, I have to be running the app. I've personally done this successfully because I visited a friend with an AppleTV and I streamed stuff from my home server via Plex, so I can see how that works. The link above is describing that process.

The thing that I think your users are looking for is something different: You turn on the TV on the AppleTV's input, and, you pick up the AppleTV remote control, and you use THAT to browse the contents of your shared library and play videos and photos and stuff. That's the part that I don't know about.

Now, I know that there is an option on Synology to have it be your iTunes Library server. They have a whole separate iTunes server app for the diskstation. Here's how it works:
https://www.synology.com/en-us/knowledgebase/tutorials/521

But the thing is, that only talks about music, not photos and videos. So I don't know.