How do I configure the Exstreamer 100 to play files from an attached USB stick/drive when the Internet goes down ?

You have to use the Streaming Client Software on the Exstreamer and check if "playlist.m3u" is set in the 3rd URL.
Copy your mp3-files into the root directory of the USB-stick.
After rebooting, the Exstreamer will find the attached USB-stick and play the music if there is no network available.

The Exstreamer with default settings will auto-create a playlist and use it.
If you want your own playlist with your playlist with your own song sequence, then create an own playlist.m3u file
and copy this also into the root of the USB-stick.

A playlist should contain only the list of audio files, one audio file name per line terminated with CR LF.
HTML formating, spaces replaced by "%20", is not supported.

To test it you simply have to copy/configure the "playlist.m3u" entry into URL1 in the setup
or you remove the network cable, in this case it will also play the USB audio even when the "playlist.m3u" entry is in URL3.

Note: There are some rules for the USB-stick. See USB_device_compatibility_list#Limitations

In the STATUS report you usually see when your USB stick is properly detected, in this case the size, the vendor and the formating system is correct recognized.