Looks like your dev has installed some security plugin/setting that protects the admin/login area.
Search for anything in SiteGround that could affect the URLs or protect the admin area.
SG Security → Login Security → “Change Login URL.”
WPS Hide Login, iThemes Security, All In One WP Security, etc.
It's very likely the URL has been changed or could be IP protected. You can disable all the plugins in WP without accessing the admin, just by moving all the plugins away from the /wp-content/plugins folder