PHP not displaying server results online - php

Question

We have built this online prototype and it worked just perfectly on our local environment, but after moving it online, some buttons and a table are not working. The buttons are all buttons that, have conditions to only show if eg. the user is logged in or has a certain user type (in this case a tutor). The table is not showing the search results.
It's possible to register a user and also login, but when entering the subpages, only logout, search button and logo is responding. And we can see that user are being saved in our database.
Can anyone give a hint to what it is we do wrong? I've displayed one of the buttons code here below.
HTML:
<ul>
<li><a class="modal-button modal-button-user" data-modal="modalEditUser"><i class="far fa-user"></i> Hello <?php echo $_SESSION["firstName"];?>!</a></li>
<li>
<form action='_home.php' method='post'><button class="logout_button" type="submit" name="logOutBtn" id="logOutBtn">Log Out</button></form>
</li>
</ul>
JS:
var modalBtns = [...document.querySelectorAll(".modal-button")];
modalBtns.forEach(function(btn){
btn.onclick = function() {
var modal = btn.getAttribute('data-modal');
document.getElementById(modal).style.display = "block"
}
});
var modalBtnsTutor = [...document.querySelectorAll(".modal-button-tutor")];
modalBtnsTutor.forEach(function(btn){
btn.onclick = function() {
var modal = btn.getAttribute('data-modal');
document.getElementById(modal).style.display = "block"
}
});
var closeBtns = [...document.querySelectorAll(".close")];
closeBtns.forEach(function(btn){
btn.onclick = function() {
var modal = btn.closest('.modal');
modal.style.display = "none";
}
});
window.onclick = function(event) {
if (event.target.className === "modal") {
event.target.style.display = "none";
}
}
PHP:
<?php
require 'config.php';
//try to make logout/login/welcomeMessage appear/disappear whether you're
logged in or not
if (isset($_SESSION["userID"])) {
echo "<script>
window.onload = function() {
document.getElementById('navUser').style.display = 'block';};
</script>";
$go = "home.php";
} else {
echo "<script>
window.onload = function() {
document.getElementById('navVisit').style.display = 'block';};
</script>";
$go = "index.php";
}
?>