Reading Software serial from Attiny85 to IDE via Arduino Uno

Hi. I am struggling to understand what I am doing incorrectly. I want to be able to read SoftwareSerial from Attiny85 via my Arduino. Seems like I am almost there, but not quite.

I have programmed Attiny85 with following:

#include <SoftwareSerial.h>
const int RX=3;
const int TX=4;

SoftwareSerial Serial(RX, TX);

void setup()
{
  Serial.begin(9600);
  Serial.println("Initializing...");
}

void loop() 
{ 

  Serial.println("Serial!");
  
}

When Attiny os powered, RX pin lights up Led, so it is sending something

Then I connected by Attiny TX pin to Arduino pin 10.

Arduino code is following:

#include <SoftwareSerial.h>
const int RX = 10;
const int TX = 11;

SoftwareSerial mySerial(RX, TX);
String Data = "";

void setup()
{
  mySerial.begin(9600);
  Serial.begin(9600);
}

void loop()
{
  while (mySerial.available())
  {
    char character = mySerial.read();
    Data.concat(character);
    if (character == '\n')
    {
      Serial.print("Received: ");
      Serial.println(Data);
      Data = "";
    }
  }
}

And I see absolutely nothing in IDE Serial monitor. Am I missing something fundamentally?

Have you joined grounds?