Hello Lorenzo,
As you mentioned, you should be able to access the Wordpress website by using the VM’s instance new IP address, and not necessarily the domain name. Since you singled the admin page, I am assuming that the actual website is already accessible with the new IP address, but not the admin page. Hence, to load the admin page, I suggest trying both combinations: [note the (s)]
http://newIP/wp-admin
https://newIP/wp-admin
Furthermore, to avoid these situations in the future, you may try assigning a static IP to the instance which should be free as long as it is in use, and if it is not in use you would be charged according to this document. I hope this helps.