index.php navigation

I think you should use wp_link_pages function, it’s was built for that purpose.

$defaults = array(
    'before'           => '<p>' . __( 'Pages:' ),
    'after'            => '</p>',
    'link_before'      => '',
    'link_after'       => '',
    'next_or_number'   => 'number',
    'separator'        => ' ',
    'nextpagelink'     => __( 'Next page' ),
    'previouspagelink' => __( 'Previous page' ),
    'pagelink'         => '%',
    'echo'             => 1
);

    wp_link_pages( $defaults );

You can refer to the codex page to learn more how to use each parameter : https://codex.wordpress.org/Function_Reference/wp_link_pages