minidlna can be used to advertise and provide a DLNA server services in the local network. Other solutions are gerbera or plexmediaserver (proprietary), which offer a nicer http interface.

The VLC media player can be used to discover existing servers on the network. For that use View -> Playlist and in the sidebar select universal plug'n'play. The DLNA server should then appear automatically, if configured correctly.

A firewall can potentially make the server undiscoverable. The UDP port 1900 has to be opened to allow incoming traffic, which is coming from 239.255.255.250, but we will skip that for this initial test to make it easier to read:

# /etc/iptables/iptables.rules
# execute $ `systemctl restart iptables` after editing
[...]
-A INPUT -p udp --dport 1900 -j ACCEPT