Apologies for the delay! Our team’s queue is longer than typical at the moment, but we’re working to get back to everyone as quickly as we can. Nuwriy is no longer a part of the support team, but I’ll be happy to help from here.
Based on the troubleshooting that’s occurred so far, we know that, on this network, things generally work okay with Displays since the Mac Mini was able to work as expected. This confirms that the problem you’re experiencing is limited to the Synology, but unfortunately we don’t have any insight into what about the Synology is blocking the connection. In the past we’ve seen Open vSwitch cause similar issues, but there can definitely be other settings or applications that could cause this behavior too — Firewalls, antivirus, VPNs; anything that might have the ability to affect network communications could be at play here.
The other thing I’ll note is that you’ll want to make sure that the Display and the Core are on the same subnet. If the Display is on a different IP range then the communication won’t work, so that’s something to check as well.