How do I change my WordPress admin log URL from ip/wp-admin to domain.com/wp-admin?

I’ve set up Wordpress in a Digital Ocen droplet.

I’m trying set up an SMTP WordPress plugin in order to send emails via Gmail. Specifically, I’m trying to set up

https://wordpress.org/plugins/gmail-smtp/

and I’m following this guide

http://www.wpbeginner.com/plugins/how-to-send-email-in-wordpress-using-the-gmail-smtp-server/

When I navigate in my browser to www.domain.com I go to my site, and the URL in the browser shows as www.domain.com.

However, when I then navigate to the wp-admin (www.domain.com/wp-admin) and log in, the URL in the browser gets changed to IP address/wp-admin.

Then when I set up the plugin and “Create a new web application and set its Authorised Redirect URL to the one shown on this page” (the first step in the process) I get the following error message in Google

Invalid Redirect: http://IP Address/wp-admin/options-general.php?page=gmail-smtp-settings&action=oauth_grant must end with a public top-level domain (such as .com or .org).

If anyone has a good idea on how to change my WordPress wp-admin log in URL to show my domain name instead would be much appreciated.

Many thanks