Set Checkbox to Checked with JavaScript after page Load

This should do what you are looking for. It works correctly in the snippet.

window.onload = onPageLoad();

function onPageLoad() {
  document.getElementById("1403317").checked = true;
}

<input type="checkbox" id="1403317">

try this one maybe ?

$(document).ready(function() { 
    $('#1403317').attr('checked', true)
};