We recommend you use WP Ultimo because it provides many features (subscription and billing feature, site creation, billing plans, pricing table, site templates, single sign on, etc.). But if you have created a WordPress multisite network without WP Ultimo, you need to be aware of these things:
Share the login session across subdomains
By default, WordPress will add the login cookie to the current domain or subdomain. For example, if you log in on the main site (site.com), you will not be logged in on the site store.site.com.
You can fix this by adding the following code to the wp-config.php file:
Change site.com with your root domain. The dot (.) before site.com means that the cookie will be shared with subdomains.
You must use the login page on the root site
You must create a login page in the main site, for example, site.com/login/. If you log in on a subsite like store.site.com/login/, the cookie might not work and the login session might not be shared with all the subdomains.
If you want to allow your customers to use custom domains in their sites, you will need to install a separate plugin for domain mapping. You can try this free plugin, but it seems to be abandoned and there is no guarantee that it will work well. The only good alternative that we can recommend is WP Ultimo.
Single sign on
If you allow your customers to use custom domains, you will need to find a single sign on plugin, so they log in on your dashboard site and automatically log in on their custom domain too. If you don’t use single-sign-on, your customers will need to login twice (on both domains). We don’t know a good standalone single-sign-on plugin.