It is actually:
- remove the existing network connection (one can do so nicely in portainer, at the very end of the container page)
- stop the container
- sudo docker network connect name_of_the_macvlan_network_adapter --ip=desired_IP_address name-of-container
- start the container
Downside: I had to do so for every of those 8 containers manually, but now I am finally presented with the login screen (not beeing able to log in, but thats an other issue).