Last night I tried to watch something from my media server on a TV that had no problem doing this before.
The media server is a DLNA server running on a Synology NAS.
It took a bit of searching before I found the solution so I’m recapping it here to help anyone else with the same problem.
The key here was that I changed my router a little while ago. The new one is an Asus mesh system.
This is where I found the solution: https://www.synoforum.com/threads/synology-nas-not-showing-up-on-your-smart-tv.5602/
The Asus app to manage the routers is fantastic, but this setting isn’t available in it. You need to access the web interface.
Go to the LAN section, then to the IPTV tab, and change the setting for “Enable multicast routing” to Enable.
As soon as I changed the setting the media server showed up in the Sources options on the TV, just as it had always done before.