I’ve only recently starting using Plex, but have seen similar issues with Sonos/Plex as well.  I was able to access my plex library from the phone and tablet (connected to local network)  but not Sonos.    If I turned off WiFi on the phone, using the mobile network, then I would lose access.  So it appears that Sonos streams your music through the plex servers, rather than directly to your local server...even though they may be on the same network.  So yes, Remote Access is required and you’ll need to paid service (someone tell me I’m wrong).

My remote access service has been rather spotty, working sometimes but not all.  It’s been working good the past few days.  I had changed a few things a week or so ago, but nothing recently.  You may want to look at plex support and forums to see if there are any known issues with your network router, what ports needs to be open, etc. 

I suspect for my case either changes needed a reboot or something to take effect, or my ISP was the issue somehow.  Don’t know.

local plex server on mac.   works perfectly on all non-sonos devices 

server has green check for remote server access.

sonos can see plex server, just not my library.

spotify casts to sonos with no problem.

i’ll play with my router for awhile.  but plex was supposed to be the workaround for the sonos 65,000 debacle.

in truth, i’m just another sonos customer ready to dumpster thousands of dollars of perfectly nice gear