I successfully deployed a website to fly.io and was able to navigate to it on the fly.dev generic url. Then I configured a subdomain with my registrar. I set an A record with the IP address I got from running the fly info command. I also configured a AAAA record pointing to the IPv6 address. Using https://www.whatsmydns.net/ I verified that these settings have propagated globally. Yet, when I try to hit the subdomain, it’s not loading the page. Seems like there must be some additional setting that I’m missing.
I tested this out to see if subdomains behaved differently than apex domains. My website was also initially not responding on my subdomains. Once I added an SSL cert for my subdomain it immediately started responding correctly. I am not sure if this is a problem with fly configuration or a more general web rule but the custom domain did not work before the ssl cert was added.