Solved! Go to Solution.
Problem was that the mdns/avahi/bonjour packets were not passed through my on my open-wrt router
After installing the respective open-wrt packages:
avahi-daemon-service-http avahi-daemon-service-ssh avahi-nodbus-daemon libavahi-nodbus-support
It works now.
Crucial is that the Spotify-Connect device is visible. On e.g. an Ubuntu system that can be checked by
$ avahi-browse -at | grep spotify + eth0 IPv4 eDMP32MB_625118 _spotify-connect._tcp local
I meanwhile switched from open-wrt to lede-project, the same applies for lede.