This is a old post and probably won’t help. however..
My process is as follows:
User Visits Website -> Selects Woocommerce Subscription Option -> Completes Payment -> User is added to Main-site & Network during this moment.
That’s when I assign the user to a website and avoid them having to pay after their account is made. There might also be some sort of setting in Ultimo to make someone pay before visiting the admin dashboard but I’ve never needed it.
I do use this process instead of the main WP Ultimo process for various reasons, but with the Ultimo+Woo integration it allows me to manage users both through Woo/Ultimo.