Thank you for the follow up @Goran_Janevski, I appreciate you taking the time to answer my questions/provide the requested feedback 
A few things moving forward…
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During your troubleshooting have you tried mounting the 205 directly to your router just to confirm what the experience is like? My assumption is that it is connected to the mentioned switch, but please do correct me if I am wrong.
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The mentioned switch is it managed or unmanaged? If it is indeed managed switch we have seen a few issues crop up with these devices in the past ( Example: #1, #2, #3). I would recommend enabling flow control on the switch, as we have seen it help other users.
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You mentioned that on the Win10 clients that they offered “two exceptions” for the “RAATServer”. If there are firewalls or antivirus active on any of your devices you will not only want exceptions for “RAATServer” but also for “Roon” as well.
Something to consider with your setup…
I noticed from your follow up that the mentioned Synology NAS is making use of an Intel Atom C2538 2.4 GHz which has the potential to cause performance issues as it is falling below our recommended specs. Does the device have an SSD installed for your Roon database?
Goran, I want to be clear that I do not believe that the behavior you are experiencing with the 205 is related to the NAS. I am merely pointing out the above as a, “head up” because I want to make sure that you get the best experience possible while using Roon, and if there is an underpowered device driving the system you may encounter limitations moving froward.
-Eric