Some DNS providers will only offer A records for root domains. Unfortunately, A records will not suffice for pointing your root domains to Heroku because they require a static IP. These records have serious availability implications when used in environments such as on-premise data-centers, cloud infrastructure services, and platforms like Heroku. Since Heroku uses dynamic IP addresses, it’s necessary to use a CNAME-like record (often referred to as ALIAS or ANAME records) so that you can point your root domain to another domain. See examples below.
heroku domains:add example.com
Adding example.com to ⬢ example-app... done
▸ Configure your apps DNS provider to point to the DNS Target
▸ whispering-willow-5678.herokudns.com.
▸ For help, see https://devcenter.heroku.com/articles/custom-domains
The domain example.com has been enqueued for addition
▸ Run heroku domains:wait 'example.com' to wait for completion