How do I use addition and || in an if else statement?

Question

This is what I am trying to accomplish but it is not working:

If the # on Dice 1 + Dice 2 equals 7, display “We have a Lucky 7!” Else If the # on Dice 1 or Dice 2 equals 6, display “We have a 6!” Else, display “We have no winner!”

This is the code I have written in javascript:

if(randomNumber1 += randomNumber2 === 7) {
document.querySelector("h1").innerHTML = " We have a lucky 7!";
}
else if (randomNumber2 || randomNumber1 === 6) {
document.querySelector("h1").innerHTML = "We have a 6! ";
}
else {
document.querySelector("h1").innerHTML = "We have no winner!";
}

What am I doing wrong?