I'm also using libreelec on a raspberry pi 3+ and 'IPTV Simple Client' worked very well with libreelec 8.2 and M3U.
When I upgraded from 8.2 to 9.0 'IPTV Simple Client' stopped working.
For some reason it seems that the installation of LE9 sets the switch 'Cashe m3u at local storage'. When deselecting that switch it works like a charm👍!