Continue playback from android Plex App to Sonos




First time poster, casual forum reader, avid user.
Noob at Plex, decent at technical stuff generally.

I am experiencing a problem which playback which I think is related to DLNA, and I would like to know if anyone has any off the top of their head info which may help me determine if it is a configuration problem which I can fix or if there is an actual bug.

The scenario :

I am out walking and listening to music in the plex android app and I arrive home. I would now like to pause the playback from my phone and continue it on my Sonos speakers.

My intuition is to use the "Connect to device" function in the app, thereby telling my phone to tell the Sonos to fetch the music from the same source that it is using and continue where I was.
The "Connect to device" menu comes up. It shows my sonos as an available device, but when selected it immediately (quicker than I think it could make a request on my network to the Sonos device) comes up with an error message saying

"Unable to connect to Sonos Room Name"

I can work around this by opening the Sonos app and selecting whatever I was listening to, but it does seem like something that ought to work.
I'm pretty sure that this is related to my configuration somehow and not an absolute failure of the functioning of the Plex app. One indication in that direction is that it works as I expect when switching the playback to the chromecast, which as it happens is connected to the Sonos (Another available workaround). But it seems like this a problem that ought to be possible to fix.

Any ideas?

My set up is basically a linux server running Plex connected by ethernet to the same network that the Sonos is connected to by ethernet.
The phone does the playback from external network, but is connected by wifi to the aforementioned network when I want the function to work.

In case this is about how the plex app refers to the server (as the external url, rather than the internal network perspective which the Sonos expects) I have tested playback in the app which was started while the phone was connected only to the wifi which the other devices is connected to (although by cable). This gives the same result.

I hope my explanation is clear, but feel free to tell me to clarify.

Thanks for any and all help!


Additional information:
There seems to be a problem on Sonos of playing back flac files with high sampling rates, but this does not seem to be the source of the problem.