How do I use a custom domain name with my compute instance?

Hi Linode Community! I'm using Spring Boot with the embedded Tomcat server, and have turned it into a Ubuntu service with Gradle/Java -jar for a war file. It works just fine through my browser via my assigned IP address, but it gives "Error 521, web server is down" when using domain name.

Steps completed after reading "Set up and Secure a Compute Instance," and "Set up Cloudflare with Linode" tutorials, plus a few Community Qs and As:

• Set up and hardened a compute instance on Akamai (a nanode).
• Purchased a domain name from Cloudflare.
• Configured my hostname in Ubuntu (domain.etc, not a FQDN).
• Updated hosts file.
• Entered the appropriate proxied A and AAAA DNS records in Cloudflare.
• Created unproxied A and AAAA DNS records on Cloudflare using a "proxy.rest-of-domain" subdomain since Akamai needs unproxied records to rDNS, and rDNS is the only way to use domain names (right?).
• Added the two Cloudflare nameservers given to my domain to the top of Linode Cloud Manager's nameservers list.
• Added proxy.rest-of-domain as rDNS on Networks tab in my nanode's Cloud Manager.

I've also:

• Tried with and without a/AAAA domains set up within the Linode Cloud Manager.
• Tried turning off Cloud Manager and Ubuntu firewalls, in case I'm not allowing it right.
• Tried using Cloudflare without any SSL as well as with Flexible SSL (which is http to origin host on Linode, anyway).
• Tried at different points to do the same thing with both Nginx and Apache2, but with the same result of IP address working but domain name failing.

I'm completely new to web administration, so if anyone could (patiently) please help me understand what I've done wrong, or not done enough, I would be most appreciative!

Do I need to play around with worker scripts? Do I need to create a different type of DNS record? Should I create identical A/AAAA records in the Akamai Cloud Manager domain page? Does Linode just not work with Cloudflare, any more? After six weeks at this, I'm at a loss for what to do.

Edit: I'm fully planning on continuing to pay for this $5/month nanode once the free trial runs out, but is the free trial somehow to blame? I don't need to pay for a bigger compute instance, do I? It works just fine using IP in browser, so I assume there's no issue with space or memory.