If your domain is registered with SiteGround, you can access your DNS Zone from your Client Area → Services → Domains→ Manage.
You can find a detailed guide on creating, editing, and deleting DNS records at SiteGround here: DNS Management.
To connect a domain with your Hostinger services, use the following values:
mx1.hostinger.com (Priority: 5)
mx2.hostinger.com (Priority: 10)
Hostinger A record - you can locate it in the Plan details section of hPanel
If you face any difficulties while managing the DNS Zone, make sure to contact SiteGround support directly. Alternatively, you can transfer your domain to Hostinger so you can manage your hosting and domain in one place.