Hi there, it’s not possible to add HTML (like line breaks) to the site title. What is possible is forcing the site title to be a specific length, which in turn would force it to break. Right now it looks like you’ve hidden your site title, but if you still need help and would like to try the method I just mentioned, please let me know.
Also just a heads-up that since your site has a Premium plan, you’re entitled to live-chat and email support, accessible here:
https://wordpress.com/help/contact