My network has a Synology NAS running a DLNA server and two Apple TV 4K devices (identical hardware, with identical firmware).
One of my Apple TV devices can reliably see the Synology DLNA device: it appears in the tvOS VLC application as well as in all other DLNA compatible applications. However, on the other device (located in a different room of the house), my DLNA devices are only listed very occasionally or rarely, so most of the time they do not appear in the tvOS VLC application, also an application of DLNA diagnostic that I place The AppleTV reports that it did not find any DLNA server in the local network.
The network topology for both Apple TVs is similar: they both connect immediately over ethernet wired to a gigabit switch that then connect to the same gigabit switch.
The network configuration seems to be the same on both Apple TVs (except for the IP address) and both use the same DNS server (a DNS server with Windows Active Directory domain controller).