local usage of SONOS without remote access



Happy new Year !

Today I just have installed the Plex Server with a Plex pass on a QNAP NAS TS-451.
I was able to configure the media libary located on the NAS to use from the Plex media app and to configure the SONOS Plex app for my iphone. But when using the plex media service on my iphone or at my windows 10 pc it shows only the name of the NAS and after that the following error message occurs:

DE unable to contact the Plex Media Server.
Please check your server settings at https://plex.tv and make
sure that Remote Access is enabled.

I really don't understand why "remote Access" is necessary at this point, because I want to use my equipment only at home in my local network. Btw. it really doesn't make any sense to use the SONOS wireless speaker from outside of my local network, does it ?

What's wrong with my setup ?
How can I solve it ?

Thanks in advance


Thanks for your infinite support of my issue ! :expressionless: