Modem Serial Passthrough

We have connected our DLMS Indian Standarad using an ESP32 and a 4G modem. The ESP32 acts as a serial passthrough i.e. it will take the data received over modem serial and forward the same to the meter. The ESP32 does not do anything other than beig a simple postman.
This code works perfectly if we have meter over serial1 and data send out by esp32 wifi
The same code only by removing wifi for sending out data and using 4g modem on serial2 is not working. we get data retry messages
Whats the issue between wifi and modem. Since esp32 is only a passthrough this should not matter.
adding code that we have created in esp32.

===========

esp32 arduino wifi working code:

#include <WiFi.h>

const char* ssid = "XXX";
const char* password = "XXXX*";

// rs port config
const int baudrate = 19200;
const int rs_config = SERIAL_8N1;

// reading buffor config
#define BUFFER_SIZE 1024

// global objects
WiFiServer wifiServer(4059);
byte buff[BUFFER_SIZE];

int size = 0;
char s[50];

void setup()
{
Serial.begin(115200);
Serial1.begin(baudrate, rs_config, 22, 23); //this is the meter on serial1
delay(10);

// We start by connecting to a WiFi network

WiFi.begin(ssid, password);

while (WiFi.status() != WL_CONNECTED) {
delay(500);
Serial.print(".");
}

Serial.println("");
Serial.println("WiFi connected");
Serial.println("IP address: ");
Serial.println(WiFi.localIP());

wifiServer.begin();

// Use WiFiClient class to create TCP connections
delay(2000);

Serial.print("Writing <<CONFIGOK>>");
Serial1.write("<<CONFIGOK>>");
// Serial1.flush();
// read data from serial and send to wifi client
while ((size = Serial1.available())) {
Serial.print("reading in setup\n");
size = (size >= BUFFER_SIZE ? BUFFER_SIZE : size);
Serial1.readBytes(buff, size);
snprintf(s, 50, "%d", buff);
Serial.print(s);
Serial1.flush();
}

delay(1000);
}

int value = 0;

void loop()
{
// delay(5000);
++value;

Serial.print("waiting for connection\r\n");
//// Serial.println(host);
//
WiFiClient client = wifiServer.available();
if (client) {

while (client.connected()) {
// read data from wifi client and send to serial
while (size = client.available() > 0) {
size = (size >= BUFFER_SIZE ? BUFFER_SIZE : size);
client.read(buff, size);
snprintf(s, 50, "%d", buff);
Serial1.write(buff, size);
Serial1.flush();
}

// read data from serial and send to wifi client
while ((size = Serial1.available())) {
Serial.print("Writing to Client\n");
size = (size >= BUFFER_SIZE ? BUFFER_SIZE : size);
Serial1.readBytes(buff, size);
client.write(buff, size);
client.flush();
}

}

client.stop();
Serial.println("Client disconnected");
delay(100);

}
}

===========

esp32 using serial 4g modem code:

#include <WiFi.h>

// rs port config
const int baudrate = 19200;
const int rs_config = SERIAL_8N1;

// reading buffor config
#define BUFFER_SIZE 1024

// global objects
//WiFiServer server;
//WiFiClient client;
byte buff[BUFFER_SIZE];
const unsigned int MAX_MESSAGE_LENGTH = 12;

int size = 0;
char s[50];

void setup()
{
Serial1.begin(baudrate, rs_config, 22, 23); //this is the meter on serial1
delay(10);

// Serial.print("Writing <<CONFIGOK>>");
Serial1.write("<<CONFIGOK>>\r\n");
Serial1.flush();
read data from serial and send to wifi client
while ((size = Serial1.available())) {
// Serial.print("reading in setup\n");
size = (size >= BUFFER_SIZE ? BUFFER_SIZE : size);
Serial1.readBytes(buff, size);
snprintf(s, 50, "%d", buff);
// Serial.print(s);
// client.write(s);
Serial1.flush();
}

delay(2000);

Serial2.begin(115200, SERIAL_8N1, 3, 1); //this is the modem on serial2
delay(100);
Serial2.write("at+creg?\r\n");
delay(1000);
// Serial.print(Serial2.readStringUntil('\n'));
Serial2.write("at+cmee=0\r\n");
Serial2.write("at+cipmux=0\r\n");

Serial2.write("AT+CGPADDR\r\n");
delay(3000);
// Serial.print(Serial2.readStringUntil('\n'));
Serial2.write("AT+CIPSERVER=1,10002,1\r\n");
delay(2000);
}

void loop()
{

// Serial.print("waiting for connection\r\n");
Serial2.write("at+cipstatus\r\n");
char response[200];
// for(int i = 0 ; Serial2.available() > 0 && i<200 ; i++) {
// response[i++] = Serial2.read();
// }
// Serial.print(response);
while ((size = Serial2.available())) {
// Serial.print("reading in setup\n");
size = (size >= BUFFER_SIZE ? BUFFER_SIZE : size);
Serial2.readBytes(buff, size);
snprintf(response, 200, "%s", buff);
Serial.print(response);
Serial2.flush();
}
if(strstr(response, "STATE:CONNECTED")) {
delay(1000);
int flag = 1;

while (flag == 1) {
// read data from wifi client and send to serial
while (size = Serial2.available() > 0) {
size = (size >= BUFFER_SIZE ? BUFFER_SIZE : size);
Serial2.read(buff, size);
// snprintf(s, 50, "%d", buff);
// if(s=="201275")
// ESP.restart();
// // Serial.println(s);
Serial1.write(buff, size);
Serial1.flush();
}
//
// // read data from serial and send to wifi client
Serial2.write("at+cipsend\r\n");
delay(200);
// Serial2.write("we got in");
while ((size = Serial1.available())) {
// Serial.print("Writing to Client\n");
size = (size >= BUFFER_SIZE ? BUFFER_SIZE : size);
Serial1.readBytes(buff, size);
Serial2.write(buff, size);
Serial2.flush();
}
Serial2.write(char(26));
delay(200);

Serial2.write("at+cipstatus\r\n");
char response1[200];
// for(int i = 0 ; Serial2.available() > 0 && i<200 ; i++) {
// response1[i++] = Serial2.read();
// }
while ((size = Serial2.available())) {
// Serial.print("reading in setup\n");
size = (size >= BUFFER_SIZE ? BUFFER_SIZE : size);
Serial2.readBytes(buff, size);
snprintf(response1, 200, "%s", buff);
Serial.print(response1);
Serial2.flush();
}
if(strstr(response1, "STATE:INITIAL")) {
flag = 0;
}
//
}
//
// client.stop();
Serial.println("Client disconnected");
delay(100);

}

delay(2000);
}

===========

Error with 4g modem using Python Client:
TX: 10:55:47 00 01 00 30 00 01 00 70 60 6E A1 09 06 07 60 85 74 05 08 01 03 A6 0A 04 08 44 4D 53 30 30 30 30 34 8A 02 07 80 8B 07 60 85 74 05 08 02 02 AC 12 80 10 01 CF 44 47 C3 D9 69 59 F2 DC 1A 5E 2F B5 A2 2F BE 34 04 32 21 30 30 00 00 00 00 4E 1F 33 40 E5 80 33 EA 4D 74 4F B7 FB 02 ED A0 F5 A0 17 2A 38 EC EE 60 AC 70 F2 E6 F7 24 1E D5 8A 96 6E 53 C4 B4 A4 26 15 83 34
Data send failed. Try to resend 1/3
Data send failed. Try to resend 2/3
RX: 10:56:02
Traceback (most recent call last):

Error with 4g modem using Java Client:
Invalid data type: 76
or Invalid data type: 40
or any other number.
===========

Thank you in advance.