Tak zkus:
(tep a vlh je uvedeneja jako h / t)

void setup()   {                
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);  //0x3C popripade nahrad tim, jak je iniciilizovan ten tvuj
  display.display(); // show splashscreen
  delay(2000);
  display.clearDisplay();  
  
  dht.begin();
  delay(2000);
}

void loop() {
  float h = dht.readHumidity();
  float t = dht.readTemperature();

  if (isnan(h) || isnan(t) {
    delay(2000);
  } else{
    // prevod  temp/hum floats do char arrays
    char temp_buff[5]; char hum_buff[5];
    char temp_disp_buff[11] = "Tmp:";
    char hum_disp_buff[11] = "Hum:";
       
    dtostrf(t,2,1,temp_buff);
    strcat(temp_disp_buff,temp_buff);
    dtostrf(h,2,1,hum_buff);
    strcat(hum_disp_buff,hum_buff);   
    
    display.clearDisplay();
    display.setTextSize(2);
    display.setTextColor(WHITE);
    display.setCursor(0,0);
    display.println(temp_disp_buff);
    display.println(hum_disp_buff);
    display.display();
    delay(2000);
  }
}