For anyone reading this and having a similar problem, I just managed to fix this, by simply purging the unity-control-center. This will change the default one to the gnome-control-center.
Since the reasons I still use unity weren't really connected to the control center this was imo the most future-proof solution.
So a simple
sudo apt purge unity-control-...
I am experiencing the exact same issue.
If you don't need to use networkd, I think your issue can be solved by adding this file /etc/netplan/01-network-manager-all.yaml with these contents to your netplan directory.
After you have added this file make sure there aren't any other files in that directory ...