1. I've got an Elegoo Mega2560 r3 board with a Sunfounder 5100 Ethernet shield.
2. There is NO radio on this board and it won't have one.
3. I am trying to configure it as a MQTT gateway without transport to be concerned with local sensors only.
4. This board/shield combo will init and get onto the network just fine using other sample basic (non-MySensors) tutorial sketches I've found, but not when running MySensors.

-> I am trying to get the board to initialize, at this point I haven't complicated the basic MQTT gateway sample sketch with any I/O sensor configs; I am simply watching the Serial Monitor to see if the board inits and gets on the network.

-> I know the Mega has potentially some configuration changes versus an Uno, but all of the threads I've researched and found are 2-3-4 years old and I don't know if things have been updated in the MySensors framework to support a Mega + 5100 Ethernet shield natively. Hopefully so!

Specific Requests:

1. If anyone could clarify what I need to change in my code to get this board/Eth shield working I would be MOST appreciative!

2. I downgraded my Boards Manager version to 1.6.11 from 1.6.23 after reading some older threads about the Mega/5100 combo, what should I set this to?

Here's my code.

/*
 * The MySensors Arduino library handles the wireless radio link and protocol
 * between your home built sensors/actuators and HA controller of choice.
 * The sensors forms a self healing radio network with optional repeaters. Each
 * repeater and gateway builds a routing tables in EEPROM which keeps track of the
 * network topology allowing messages to be routed to nodes.
 *
 * Created by Henrik Ekblad <henrik.ekblad@mysensors.org>
 * Copyright (C) 2013-2019 Sensnology AB
 * Full contributor list: https://github.com/mysensors/MySensors/graphs/contributors
 *
 * Documentation: http://www.mysensors.org
 * Support Forum: http://forum.mysensors.org
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 as published by the Free Software Foundation.
 *
 *******************************
 *
 * REVISION HISTORY
 * Version 1.0 - Henrik Ekblad
 *
 * DESCRIPTION
 * The W5100 MQTT gateway sends radio network (or locally attached sensors) data to your MQTT broker.
 * The node also listens to MY_MQTT_TOPIC_PREFIX and sends out those messages to the radio network
 *
 * LED purposes:
 * - To use the feature, uncomment WITH_LEDS_BLINKING in MyConfig.h
 * - RX (green) - blink fast on radio message received. In inclusion mode will blink fast only on presentation received
 * - TX (yellow) - blink fast on radio message transmitted. In inclusion mode will blink slowly
 * - ERR (red) - fast blink on error during transmission error or receive crc error
 *
 * See http://www.mysensors.org/build/esp8266_gateway for wiring instructions.
 * nRF24L01+  ESP8266
 * VCC        VCC
 * CE         GPIO4
 * CSN/CS     GPIO15
 * SCK        GPIO14
 * MISO       GPIO12
 * MOSI       GPIO13
 *
 * Not all ESP8266 modules have all pins available on their external interface.
 * This code has been tested on an ESP-12 module.
 * The ESP8266 requires a certain pin configuration to download code, and another one to run code:
 * - Connect REST (reset) via 10K pullup resistor to VCC, and via switch to GND ('reset switch')
 * - Connect GPIO15 via 10K pulldown resistor to GND
 * - Connect CH_PD via 10K resistor to VCC
 * - Connect GPIO2 via 10K resistor to VCC
 * - Connect GPIO0 via 10K resistor to VCC, and via switch to GND ('bootload switch')
 *
  * Inclusion mode button:
 * - Connect GPIO5 via switch to GND ('inclusion switch')
 *
 * Hardware SHA204 signing is currently not supported!
 *
 * Make sure to fill in your ssid and WiFi password below for ssid & pass.
 */


// Enable debug prints to serial monitor, baud rate is 115200 for console
#define MY_DEBUG

// Enables and select radio type (if attached)
//#define MY_RADIO_RF24
//#define MY_RADIO_RFM69
//#define MY_RADIO_RFM95

#define MY_GATEWAY_MQTT_CLIENT

// Set this node's subscribe and publish topic prefix
#define MY_MQTT_PUBLISH_TOPIC_PREFIX "hvac/dampers"
#define MY_MQTT_SUBSCRIBE_TOPIC_PREFIX "hvac/dampers"

// Set MQTT client id
#define MY_MQTT_CLIENT_ID "mysensors-board1"

// Enable gateway ethernet module type
#define MY_GATEWAY_W5100

// W5100 Ethernet module SPI enable (optional if using a shield/module that manages SPI_EN signal)
#define MY_W5100_SPI_EN 4

// Enable Soft SPI for NRF radio (note different radio wiring is required)
// The W5100 ethernet module seems to have a hard time co-operate with
// radio on the same spi bus.
//#if !defined(MY_W5100_SPI_EN) && !defined(ARDUINO_ARCH_SAMD)
#define MY_SOFTSPI
#define MY_SOFT_SPI_SCK_PIN 52 //14 was default, changed to test Mega board
#define MY_SOFT_SPI_MISO_PIN 50 //16 was default, changed to test Mega board
#define MY_SOFT_SPI_MOSI_PIN 51 //15 was default, changed to test Mega board
//#endif

// When W5100 is connected we have to move CE/CSN pins for NRF radio
#ifndef MY_RF24_CE_PIN
#define MY_RF24_CE_PIN 5
#endif
#ifndef MY_RF24_CS_PIN
#define MY_RF24_CS_PIN 6
#endif

// Enable these if your MQTT broker requires username/password
//#define MY_MQTT_USER "username"
//#define MY_MQTT_PASSWORD "password"

// Enable MY_IP_ADDRESS here if you want a static ip address (no DHCP)
#define MY_IP_ADDRESS 10,2,20,24
#define MY_MAC_ADDRESS 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED

// If using static ip you can define Gateway and Subnet address as well
#define MY_IP_GATEWAY_ADDRESS 10,2,20,1
#define MY_IP_SUBNET_ADDRESS 255,255,255,0

// MQTT broker ip address or url. Define one or the other.
//#define MY_CONTROLLER_URL_ADDRESS "m20.cloudmqtt.com"
#define MY_CONTROLLER_IP_ADDRESS 10, 2, 10, 34

// The MQTT broker port to to open
#define MY_PORT 1883

/*
// Enable inclusion mode
#define MY_INCLUSION_MODE_FEATURE
// Enable Inclusion mode button on gateway
//#define MY_INCLUSION_BUTTON_FEATURE
// Set inclusion mode duration (in seconds)
#define MY_INCLUSION_MODE_DURATION 60
// Digital pin used for inclusion mode button
//#define MY_INCLUSION_MODE_BUTTON_PIN  3

// Set blinking period
#define MY_DEFAULT_LED_BLINK_PERIOD 300

// Flash leds on rx/tx/err
// Uncomment to override default HW configurations
//#define MY_DEFAULT_ERR_LED_PIN 16  // Error led pin
//#define MY_DEFAULT_RX_LED_PIN  16  // Receive led pin
//#define MY_DEFAULT_TX_LED_PIN  16  // the PCB, on board LED
*/

#include <Ethernet.h>
#include <MySensors.h>
#include <Bounce2.h>

void setup()
{
    // Setup locally attached sensors
}

void presentation()
{
    // Present locally attached sensors here
}

void loop()
{
    // Send locally attached sensors data here
}

Here's what pops in the Serial console:

10:41:18.848 -> 378897 GWT:RMQ:MQTT RECONNECT
10:41:19.428 -> 379460 GWT:TPC:IP=0.0.0.0
10:41:20.421 -> 380463 GWT:RMQ:MQTT RECONNECT
10:41:21.003 -> 381026 GWT:TPC:IP=0.0.0.0
10:41:21.994 -> 382028 GWT:RMQ:MQTT RECONNECT
10:41:22.579 -> 382592 GWT:TPC:IP=0.0.0.0
10:41:23.568 -> 383594 GWT:RMQ:MQTT RECONNECT
10:41:24.146 -> 384157 GWT:TPC:IP=0.0.0.0
10:41:25.137 -> 385160 GWT:RMQ:MQTT RECONNECT