Here's a handful of streaming links to what you're looking for. I don't know if they're underground per se but they play more obscure stuff often.
Frequence 3 - http://ice1.ikoula.net-radio.fr/frequence3urban.flac.m3u
http://trap-high.rautemusik.fm/
https://ice55.securenetsystems.net/DASH8
Lookup Radio Meuh from France and Dash Hip-Hop X from California