What error are you getting? Because for us it seemed that the Cloudflare <-> HerokuDNS link was down. But even then I was able to access my app from the dashboard (like https://my-app.herokuapp.com/)
1. Go to your Heroku app, copy the DNS-target for your apex (root) domain (the one that looks like "stark-horse-mrp4jeowu9yvwpnnma32x6hd.herokudns.com")
For me the securitytrails.com website just crashes. I put my DNS target: "stark-horse-mrp4jeowu9yvwpnnma32x6hd.herokudns.com", clicked "Run Check" and it seems to redirect to a failed (no CSS) webpage. Anyone else experiencing this?
Make sure to go through all the tabs at the top (Cloudflare DNS, Google DNS) - for me they were all "no A records found". Only "Authoritative" gave me 3 A records which I successfully managed to use.