I found the answer. I had searched high and low yesterday, but today I finally found it: https://wordpress.stackexchange.com/a/143837/10595
add_action('template_redirect', function() {
if ( is_singular('posttype') ) {
global $wp_query;
$page = (int) $wp_query->get('page');
if ( $page > 1 ) {
// convert 'page' to 'paged'
$query->set( 'page', 1 );
$query->set( 'paged', $page );
}
// prevent redirect
remove_action( 'template_redirect', 'redirect_canonical' );
}
}, 0 ); // on priority 0 to be able removing 'redirect_canonical' added with priority 10
This, copied to my functions.php, makes WordPress recognize the paged
parameter on single posttype
entries.