WebRTC variable bitrate

Hi!

I have set up a Janus media server, and currently have a working conference call.
I noticed that when I have lower available bandwidth, my PeerConnections get disconnected and must be renegotiated.
From looking into Opus, I thought it should have variable bit rate - is there any change I should make to the Janus config or to my peer connection parameters to enable the variable bitrate?

Can you please help me with understanding how to support lower bandwidth?

Thanks!