If your domain is registered with DigitalOcean, you can access your DNS Zone from your Control Panel → Networking tab.
You can find a detailed guide on how to manage DNS records at DigitalOcean here: How to Create, Edit and Delete DNS Records.
To connect a domain with your Hostinger services, use the following values:
mx1.hostinger.com (Priority: 5)
mx2.hostinger.com (Priority: 10)
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 DigitalOcean support directly. Alternatively, you can transfer your domain to Hostinger so you can manage your hosting and domain in one place.