Networking: how to sync complex objects

Hi, I'm new with the Networking classes, but I'm starting to understand how the syncing between client and server works. You have to extend NetworkBehaviour and add [SyncVar] before each synced property.

However I don't understand how to do the following:

I have a MissionsManager class, which extends Singleton, so it can't extend NetworkBehaviour. Also my MissionsManager has a List, and my understanding is that you can only use SyncVar on basic types, which List is not.

I tried to add a Network Identity to my MissionsManager, but that didn't change anything. From what I understand it's only meant for objects that are spawned. But my MissionsManager is not spawned, it's just there.

How do you sync those kind of things? Is the networking api at all meant for this?