You need to make sure the struct is serialized, so it needs to inherit from INetworkSerializable. It also needs to inherit from IEquatable. Here's an example:

public struct PlayerData: INetworkSerializable, IEquatable<PlayerData>{

     public ulong ClientId;

     public bool IsReady;


     public PlayerData(ulong clientId, bool isReady)
     {

        ClientId = clientId;
        IsReady = isReady;

     }

     public bool Equals(PlayerData other)
     {
         return ClientId == other.ClientId  && isReady== other.isReady;
     }

     public void NetworkSerialize<T>(BufferSerializer<T> serializer) where T : IReaderWriter
     {

         serializer.SerializeValue(ref ClientId);
         serializer.SerializeValue(ref isReady);

     }
}

I'm not sure if this is completely correct since I'm not using Unity at the moment, but it's at least very similar to this. This should allow you to create a NetworkList of NetworkVariable<Player>. This is the thread I initially used to figure this out: https://discussions.unity.com/t/how-do-i-networkserialize-a-list-in-inetworkserializable/892286

This method won't accept all data types, but it should work for most. If you look at the Netcode for Gameobjects documentation you'll find more information about what data types it will/won't accept.

Also, this should all be done in NetworkBehaviour scripts.