Kubeternetes CNI Flannel on dual-homed nodes

During my quest to move to Kubernetes I seem to run into one issue after the next. Since I was unable to get Calico to work at all due to it’s inability to cope with nftables, I moved to Flannel. That one gave me some problems as well, which I however was able to fix.
Basically the issue here is that flannel does not cope with dual-homed hosts. Dual-homed means one has multiple networks attached. In my legacy cloud I did that for enhanced security. Basically all services only talk internally on a network not accessible from the internet. Saves a lot of trouble.
However the CNI plugins in Kubernetes are quite complicated when it comes to firewalls and cannot cope with dual-homed hosts.
I removed the second network and now all starts flowing.

Happy coding!