Hi, @tomtoms !
Welcome to Shopify Community.
Thanks for connecting with us and sharing your question. Domain DNS settings are only present within Shopify when the domain itself was purchased through Shopify. Otherwise, the domain DNS settings will be available through any third-party domain provider from which you purchased the domain.
Can you let me know where you purchased your domain and what DNS records you are trying to change? That way I can definitely provide further guidance.
Warm regards,