I'm working on a project that use a Teesny 3.2, 3 RFID MFRC522 and the WIZ820io module with the adaptor (waiting for the WIZ850io module to test).
I need to read the value from the three RFID and comunicate if each one is reading the correct one or another one.
The communication is in Modbus, ad I'm using the following library:

My code is the following (hiding that is not useful):
#include <SPI.h>
#include <Ethernet.h>
#include <MFRC522.h>
#include <Mudbus.h>
#define SENNUM  3 //total amount of sensors

uint8_t mac[] = {0x04, 0xE9, 0xE5, 0x06, 0xE1, 0x28}; 
uint8_t ip[] = {10, 0, 0, 108};                           //The ip is fixed

//Modbus Registers Offsets (0-9999)
const int SENSORS[SENNUM] = {1, 2};

int sensStatus[SENNUM] = {0, 0, 0};

//Used Pins
const int sensPins[SENNUM] = {14, 15, 16}; // // SS pins, Configurable

//ModbusIP object
Mudbus Mb;

// game spec initializations
constexpr uint8_t RST_PIN = 17;          // Configurable, see typical pin layout above

MFRC522 mfrc522[SENNUM];  // Create mfrc522b instances

const int nOfTAG = 8; //TAGs
String currentTAG[SENNUM];
String correctTAG[nOfTAG] = {"01836014245", "01357614245", "01674912245", "02154412245", "01349135218", "0141176138218", "07792135218", "077119144218"};

void setup() {
  // reset for Ethernet Shield
  pinMode(9, OUTPUT);
  digitalWrite(9, LOW); // reset the WIZ820io
  digitalWrite(9, HIGH); // release the WIZ820io

  pinMode(10, OUTPUT);
  digitalWrite(10, HIGH);
  pinMode(14, OUTPUT);
  digitalWrite(14, HIGH);
  pinMode(15, OUTPUT);
  digitalWrite(15, HIGH);
  pinMode(16, OUTPUT);
  digitalWrite(16, HIGH);

  Ethernet.begin(mac, ip);
  // disable w5100 SPI

  Serial.print("server is at ");

  //RFID initializzation 
  for (uint8_t i = 0; i < SENNUM; i++) {
    mfrc522[i].PCD_Init(sensPins[i], RST_PIN);   // Init mfrc522a
    mfrc522[i].PCD_DumpVersionToSerial();  // Show details of PCD - mfrc522a Card Reader details
    Serial.println(F("Scan PICC to see UID, SAK, type, and data blocks..."));

void loop() {

void gameUpdate() {
  // Look for new cards
  for (uint8_t i = 0; i < SENNUM; i++) {
    String readRFID = 0;
    if ( mfrc522[i].PICC_IsNewCardPresent() && mfrc522[i].PICC_ReadCardSerial()) {
      for (int b = 0 ; b < mfrc522[i].uid.size; b++) readRFID.concat(mfrc522[i].uid.uidByte[b]);
    if (readRFID != currentTAG[i]) {
      currentTAG[i] = readRFID;
    //Halt PICC
    // Stop encryption on PCD

  for (uint8_t i = 0; i < SENNUM; i++) {
    boolean found = false;
    // controllo dei valori dei tag sui due lettori RFID
    for (uint8_t y = 0; y < nOfTAG / 2; y++) {
      if (currentTAG[i].equals(correctTAG[y + (i * nOfTAG / 2)])) found = true;
    if (found) {
      sensStatus[i] = HIGH;
      if (!i) {
        Serial.print("Correct TAG-A ");
      else {
        Serial.print("Correct TAG-B ");
    } else  {
      if (currentTAG[i] != "0") {
        sensStatus[i] = 2;
        if (!i) {
          Serial.print("TAG-A ");
        else {
          Serial.print("TAG-B ");
      } else sensStatus[i] = LOW;
    Mb.R[SENSORS[i]] = sensStatus[i];
The circuit is this one:
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The length of the cables (Ethernet CAT 5) is approximatively 50cm each
I tried this configuration with a Teensy LC and jumper cable and it is all working.

Then I moved it with the other cable in the final room and I have ip problem. The Ethernet library show me my ip as
If I disconnect The teensy from the circuit it takes the correct ip.
If I connect one RFID only it takes the correct ip.
When I connect the second RFID initially it takes the correct ip but sometimes the device change it in or (i use this code in the loop to control it)
Serial.print("server is at ");
One strange thing is that If I connect two RFID and connect only the cable of the third, without the device, I have the same problem ( ip)

can it be an SPI problem (some conflict between the Ethernet module and the RFID modules)?

Must I add the pull-up resistor on the CS pins? i tried with 10k with no results
Must I reduce the clock frequency of the SPI? how?

looking at this link I do not understand how to integrate the Step 3: USB SPI Transactions in Software, in my software.
also I bought some 74HC4051 to use it as suggested here but I'm not sure the problem is the RFID, must I connect also the ethernet module to the multiplexer?

Any suggestion is appreciated!