Hi!
Faced the same trouble.
At this stage I just managed to get freeswitch webrtc running, cert was from letsencrypt, instructions I got from https://freeswitch.org/confluence/display/FREESWITCH/WebRTC, important moment - in FusionPbx there is a file /etc/freeswitch/tls/dtls-srtp.pem - wss did...