If your domain is registered with 123 Reg, you can access your DNS Zone from your Domain name overview → DNS tab.
You can find detailed tutorials on creating, editing, and deleting DNS records at 123-reg here: DNS Management Guide
Hostinger DNS records:
If you face any difficulties while managing DNS Zone at your domain provider, it’s best to contact them directly. Alternatively, you can transfer your domain to Hostinger so your hosting and domain will be managed from one place.