you sad u have access to phpmyadmin.
Go to it, find wp_options table
Look for custom_wpadmin_slug in option_name and in option_value column will be your’s custom defined wp-admin slug
If you’ll still wount be able to access then remove next lines in .htaccess:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^{your_wp_admin_slug}/?$ {your_site.com/}wp-login.php [QSA,L]
</IfModule>
Note that {your_wp_admin_slug} is equal to one in wp_options table