Getting 'Not Found' on custom subdomain on Netlify site. Using Route53 with Netlify and subdomain hosted on Heroku
Found it! Figures that it would click 30 minutes after posting.
I had a CNAME on Route53 but this was useless since I was using the Netlify DNS. You'll need to create a CNAME on Netlify that points to the host Heroku gives you.
So my heroku domains gave me the host I needed the CNAME to point to. I created a CNAME record on Netlify - which was not very obvious, I had to click into the badge 'Netlify DNS' in order to find the config.
I was able to create the right CNAME and then have it all work after that.