Nextcloud, its Audio-App und Sonos Plugin

NethServer Version: 7
Module: Nextcloud 19

Hi

I am running Nextcloud on Nethserver. One of the installed Apps is “audio player” with Sonos plugin,

https://apps.nextcloud.com/apps/audioplayer
https://apps.nextcloud.com/apps/audioplayer_sonos

I do not get my sonos from within Nextcloud at work. The Nextcloud / Nethserver IP is of course in the same network as the sonos players are. Other users seem to have the problem:

Obviously it seems to be a matter of firewall. So I do have two 1/2 questions

1.) Does anybody run the same piece of software? And did anybody get it work?
2.) How would I set the firewall rules within nethserver - up to know I did not have to set any firewall rules on the nethserver side. Maybe someone can show a screenshot from the respective side / how settings would be correct to open ports?

Thank you and best regards
Thorsten