WordPress Multisite blogs not working after domain swap

Have you copied and modified the specified lines from your wp-config.php from old site to the new site?

The code should look something like this

define( 'MULTISITE', true );
define( 'SUBDOMAIN_INSTALL', true );
$base="https://wordpress.stackexchange.com/";
define( 'DOMAIN_CURRENT_SITE', 'yourdomain.com' );
define( 'PATH_CURRENT_SITE', "https://wordpress.stackexchange.com/" );
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );