To restrict access to your /wp-admin login you need to have access to your .htaccess file. If you are using a host that has cPanel or DirectAdmin, you can easily do this from within there.
If your host does not allow you access to your files directly, you can install a plugin call File Manager. This is an excellent plugin that will allow you access to your files on your hosting account from within WordPress.
Locate your .htaccess file (it’s normally in your /public_html folder). Open the file and add the code below to the bottom of the file
<Files wp-login.php> order allow,deny deny from all allow from xxx.xxx.xxx.xxx allow from xxx.xxx.xxx.xxx </Files>
Replace the xxx.xxx.xxx.xxx with the IP address you wish to have access. If you need more than two IP’s, you can just add another line above the </files> part.
If you don’t know how to get your external WAN/Public IP, you can just type in google what is my IP or use this link from google.