For all how wants to do a local installation without having an DNS server, but using a Fritz Box.
I’ve used www.goip.de to get the 4 needed sub domains linked to the local ip address of my openbalena server.
I just needed to enter these domains into the DNS rebind protection exception list of the Fritz Box.
By doing this I was able to get my first device connected. (It’s still offline, but this seems to be another issue other people also have as described in Devices recognized by server but always offline)