WordPress - Put a full-page type slider into the background

If your theme doesn’t have anything else in there, then you could put your slider into the background to be in a fixed position behind everything with this code, just put it right after the <body> part of your website:

probably into the header.php file of the theme

<div style="position:fixed;top:0;bottom:0;left:0;right:0;z-index:-1;">
<?php echo do_shortcode('[smartslider2 slider="2"]'); ?>

Troubleshooting if the slider is there, just your template/theme hides it: give a bigger (positive) z-index to it. If the slider is there, just something hides it, you could try to modify your theme like this.