If you purchased a domain with Hostinger, and want to use it with external service or provider, you can easily do it by pointing your domain to that service. The way to point domain will be different based on the service itself, please check these articles to find our details:
If you have not found your provider in the list, kindly contact them to find out how to point a domain to them. On Hostinger's part you will need to do these changes:
If your domain is pointing elsewhere by NS records, your DNS Zone management is moved to the provider you pointed the domain to and should be managed from there.
Changes to your domain's nameservers will need 24 hours to fully propagate worldwide. Your website may experience issues during this period.