Pro Mini 3.3V/8 TTL not communicating with SIM7600x core board

[Ashton]

What might prevent mini pros (3.3V/8Mhz) from communicating directly (Tx-3/Rx-4) with a SIM7600x core board(rated for 3.3V ttl)? 9600 baud used.
UNO and Nano boards work perfectly using a TTL level shifter.

[lastchancename]

No code, no schematic.

[Ashton]

Hope this helps clarify.

Test code:

#include <SoftwareSerial.h>

//SIM core board Tx & Rx is connected to Arduino 3 & 4
SoftwareSerial gsmSerial(3, 4); //(TX, RX)

void setup()
{
  //Start serial communication
  Serial.begin(9600);
  gsmSerial.begin(9600);

  Serial.println("Initializing...");
  delay(1000);

  gsmSerial.println("AT"); //Basic AT command
  updateSerial();
  gsmSerial.println("AT+CSQ"); //GSM signal strength. 0-31 , 31 is the best
  updateSerial();
  gsmSerial.println("AT+CCID"); //Read SIM card information
  updateSerial();
  gsmSerial.println("AT+CREG?"); //Check if the module is registered to a network
  updateSerial();
  gsmSerial.println("AT+CBC"); //Charging status and remaining battery capacity
  updateSerial();
  gsmSerial.println("AT+GSV"); //Product information
  updateSerial();
}

void loop()
{
  updateSerial();
}

//Method for serial communication
void updateSerial()
{
  delay(500);
  while (Serial.available()) 
  {
    gsmSerial.write(Serial.read());
  }
  while(gsmSerial.available()) 
  {
    Serial.write(gsmSerial.read());
  }
}