Documenting an issue and possible solution for anyone else with the same, and possibly report a bug.
I have the Arcadyan WiFi hub (fw v3.00.21 build02) for PureFibre home internet service. The hub has a USB 3.0 port. It is possible to install a USB storage drive with videos/media on it, and configure the hub to act as a DLNA media server, as well as FTP or network share. This is a great all-in-one home networking device.
Problem I had with it, the DLNA server could not reliably be discovered. I use a Roku TV and multiple android devices with VLC app installed. Each would only discover the hub DLNA server randomly, around 1 in 20 attempts. This was regardless of any other devices connected/streaming, rebooting, IP address, disabling FTP/shares, or wireless extender.
The solution I have stumbled on is go to the Network->IPv6 page in the hub configuration, and disable IPv6 for both WAN and LAN use.
Turns out my solution isn't any good. My DLNA server is currently discovered ~50% of the time from VLC on Android, and very rarely from Roku TV. If anyone has any way to access deeper configuration of the wifi hub, please let me know.
It seems to be reliably discoverable only after I apply settings on the hub that affect it, like forcing IP address change or disabling and re-enabling the media server. This is not a viable workaround for everyone in the household, so I am still looking for a fix. Any suggestions welcome!