Need support for Synology NAS DS214play?

Salü together, maybe someone can help me technically here.

I first list my devices that I need and describe the problem later.

NAS Synology DS214play connected to the router via Ethernet
Fritzbox router, which depends on a 1GB (up / down via the provider Fiber7)
Samsung smart TV
AppleTV (4th generation 4K), connected to the router via Ethernet and via HDMI to the TV

Installed on AppleTV: DS-Video from Synology and the VLC player.

Problem: If I stream video files, I access the NAS via AppleTV via the DS-Video app. Most of the time it works. However, if I try to stream FullHD content, I often get the message "Format can't be played". I then switch on AppleTV from the DS-Video app to the VLC player, access the NAS from there and can play this video without errors via the subfolders of the NAS, which just gave me the format error via DS-Video. As a rule, these are .mkv video containers.

Does anyone have any idea why the DS-Video doesn't work on the TV, but it does on the VLC player? By the way, I could install DS-Video and VLC directly on my SmartTV and omit AppleTV as an adapter, the result is the same.

As a comparison: I also access the NAS from outside the home network via the mobile phone or tablet while on the move, also use DS-Video and then never have any problems THERE. Even files that give me the format error with DS-Video on the TV set are played perfectly on cell phones, tablets and laptops (whether internal or external to the home network is irrelevant).

Does somebody has any idea?

Is mostly a codec problem. I had this problem with DS-Video very often, which is why I have been using Plex for years.

I've already thought about Plex. The problem is currently my lack of knowledge: I would then also solve a "subscription". There's a lifetime for around 120 euro. What has kept me from doing so far is the question of whether this can solve the problem or whether it may be due to the hardware of the existing components. Should then be because of the NAS or AppleTV. The TV set is only used as an output medium.

Did you take a look at switching to Plex?

So I use Plex a bit longer than I use Synology. When I got the first disk station ~ 6 years ago, I of course tried DS Video because I thought that a piece of hardware would work better with software from my own company. But then very often had the problem that films could not be played back.

Then I installed Plex when it was available and it worked fine. In addition, the usability was just better.

However, I have a slightly larger disk station than you. I have 4 GB of RAM while yours only has 1 GB.

But you can just try it out. You don't have to subscribe right away.