Turtle Pass is an addon for Fishnet that allows you to send large byte arrays over several frames so it doesn't overwhelm more limiting transports like FishySteamworks and FishyUtp

Add Turtle Pass Manager component to the NetworkManager. The Debug bool can log relevant Turtle Pass messages to console when toggled.

To send a large byte array call:

TurtlePassManager.QueueSendBytes(int senderID, TurtlePassDataType dataType , byte[] data, int packedSize, bool sendToServer = false, int ToSpecificID = -1)

• int senderID - Use the id of the sender, -1 for server.
• enum TurtlePassDataType - It's an enum. You can add your own message types to TurtlePassDataType by editing the enum. It identifies the type of byte array so the receiver knows what to do with it.
• byte[] data - This is your large byte[]
• int packedSize - The length of your byte[] or another int that might help the receiver handle your byte[].
• optional bool sendToServer - Set to true if you're sending this to the server as a client, it's optional and if it's set to false it'll assume this is a server sending to clients.
• optional int ToSpecificID - If it's set to a number different than -1 it'll assume you're trying to send to a specific client and it'll only send the message to the client with the corresponding connecion id. (Only works if you're sending as a server).

On the script you want to receive the message implement the interface ITurtlePassReceiver

public interface ITurtlePassReceiver {
    public void ReceiveTurtlePassMessage(byte[] data,int packedSize, int senderId, TurtlePassDataType dataType);
}

Then add your receiver to the Receivers list in the Turtle Pass Manager

        TurtlePassManager.Receivers.Add(this);

It'll then call the ReceiveTurtlePassMessage method in your receiver when the message has been fully received.

Sending rates are controlled by 3 const values in the beginning of Turtle Pass Manager

    public const int maxSize = 86000; //Maximum size of the message in bytes
    public const int maxSendPerAttempt = 1; //How many messages to send in a single pass.
    public const float sendInterval = 0.33f; //Time interval between passes.

These values worked well for me but feel free to play around with them.