WP SESSIONS not remembering values on remote server

locally and on my personal remote server, functions.php w the following:

if (isset($_GET['define_ses'])) {
	$_SESSION['test'] = 'defined';
} else {
	print_r($_SESSION);
}

works. Ie:, I define the session w the URL https://.../?define_ses , then change the URL and on every page, $_SESSION['test'] = 'defined' as expected. the value is remebered.

BUT on my clients remote server, this is not the case…

When the URL is https://.../?define_ses $_SESSION['test'] = 'defined'
but when the url changes the var is blank $_SESSION['test'] = '' . the value is not being carried from page-to-page as it is on my local and personal server.

The only thing I know about this server is that “Mod_security is in effect“. I know it’s wordpress because when i put that first snippet into a non-wordpress test.php page, the session’s do work and it is remembered- it’s only not remembering in my wordpress files.

Can anyone please let me know any reason why my session values aren’t being remembered?