question

I seem to recall reading before that if both Ethernet and WiFi are connected at the same time, that the cerbo will automatically prefer the Ethernet connection for internet/VRM. I don't believe there is any option to change this behavior, unless you are intimately familiar with Linux, in which case you might be able to do something under the hood. Are you able to connect your peripheral devices via WiFi instead of Ethernet? That would be a viable solution. If you are just concerned about not having them on a network that is accessible from the internet you could connect them to the built in wifi access point instead.

The internal Ethernet is the one from the ship where more or less critical technical devices are connected. No way to connect the cerbo through WiFi with that network. (Sure I could use a WiFi to Ethernet bridge, but I would like to avoid that!)

I really don’t want internet uplink there.

On the WiFi - where the cerbo is connected to - is internet uplink and all smartphones / entertainment devices and so on…

I’m happy to change things through ssh, just need to know what kind of settings survives reboots and updates :)

See 6.7 of the Cerbo manual.

When both present with internet, will use the ethernet. Otherwise searches the interfaces to see which has internet and uses that.

So your problem is that it's not seeing the internet. Or the WiFi connection. Maybe disconnect ethernet, configure test using WiFi only and then reconnect ethernet and see what happens.

Best make sure there's no bridge via the Cerbo between the networks.