So some time have passed including Christmas since the question was asked and there still seem to be no answer on how to change or make the router settings connecting the wifi Sonos use the LAN address rather than the WAN when connecting to Plex, if that is what I understand the route of the problem really is of Sonos not playing the visible Plex library. If a knowledgeable techy member or Plex support could perhaps explain in simple language how to change the router config or whatever settings need changing to make Sonos play the music, I and it seems several other members would be very grateful!
Before Christmas, I had several email chat conversations with Sonos support on this problem but no working solution was found, so even the experts seem baffled by this! So if anybody has got a working fix for this then they are up for a gold star!
I also have an even bigger anomaly with this situation and that is when my internet connection is going through my VPN the Sonos reads the Plex library and PLAYS the music too, also the artwork pictures are displayed, but only when connected through my VPN, so can anybody tell me why this should be? And like others on here, the Plex libraries work fine through the Smart TV sound bar and other playing devices, just the Sonos has the problem with playing the Plex music!