UPnP/DLNA

UPnP is a standard allowing all kind of devices to communicate with each other over the network.

UPnP (Universal Plug and Play) is a bit of a pun on Plug and Play.
UPnP compliant devices will automaticalle detect each other in the network.
They exchange their capabilities so they know what each of them can do.

Often routers support UPnP.
This is UPnP over the internet (WAN).

You can access your media at home over the the internet.
Unfortunately it is a security risk as you have to open ports in your router.

Using a VPN connection is an alternative.

You don’t need this option to do UPnP over the LAN as inside the LAN the router works as a switch and simply passes all traffic from one device to another including UPnP/DLNA.

If you don't need access to the media inside your LAN over the internet, disable this option on your router.

Issues

Reliability

A couple of years ago UPnP/DLAN was most of all Plug and Pray.

Today most DLNA implementations are mature.
A lot of smart TVs and smart phones support this protocol.
If possible try to find out if the device is DLNA certified.

Gapless playback

As far as I know gapless playback is not part of the DLNA standard.
UPnP does (the magic word seems to be the SetNextAVTransport).
Always check if gapless is supported (some streaming audio players do).

Nothing as annoying as a white space in a live concert.

Transcoding

Devices do exchange their capabilities.
A playback device might report it is capable of playing WAV and MP3.
What does the server sends to the playback device?
They might simply decide to save some bandwidth and convert everything to MP3.
As a user you often don’t have any option to control this.

Servers like Asset UPnP or MinimServer supports transcoding.

This allows you to specify the format send to the playback device.

Tags

The common tags like Album, Artist, Title, Genre are supported.
Sometimes you can configure what tags are displayed.
A server like MinimServer displays the Composer, Conductor, Orchestra and Soloist tags.
As far as I know, it is in not possible to display custom tags.