There are a number of similar issues reported - and answered, so I can’t add my twopenn’orth, so here it is.

I have large Sonos system and an extensive music library on a Synnas DS920+ with Plex. It has proved troublesome, and after moving from BT to EE, replacing the router, became more so. Playing became more intermittent, with ‘The connection to Plex was lost’ randomly appearing. One day suddenly it ceased to function alltogether and I raised case 06921049 with support (I haven’t managed to get back to support to tell them what the solution is yet). After hours of Sonos chat and diagnosis it eventually started working again without anything obvious having been changed, but was still randomly intermittent.

I have now discovered that the answer is switching off uPNP support on the new EE router (which is similar but different to the old BTHub which allowed more flexibility in setup, and seemed to be much more responsive on the web interface. The EE kit is a retrograde step from the BT all round it seems). I had to manually set up a port forwarding rule for any remote access to my Plex server. Now the Sonos playback is seamless… Sonos playback does NOT require the remote forwarding rule - for some time I was in two minds as to whether the issue was Sonos accessing it remotely rather than internally.

It would seem that there is a mutually incompatible flaw between EE, Sonos, Synnas and Plex. I never could get Sonos to see a DNLA server on the Synnas - I might try that again now.