Cloudflare Keeps Saying 521 Server is Down

I've followed the tutorial on setting up Cloudflare as the DNS for my Linode host (https://www.linode.com/docs/guides/how-to-set-up-cloudflare-with-linode/ ) and have entered the proper allowances for Cloudflare in both the Linode Manager firewall as well as UFW within the host OS. For this I've allowed 1.1.1.1 and 1.0.0.1 to and from anywhere, and have both set to allow to ports 53 and 8080 from anywhere, anyway. I'll restrict it further to proper ports and protocols once I get it working in the first place. I've also disabled the manager and UFW firewalls, to see if I wasn't allowing it right. I have my hosts file configured as defined in your setup tutorial (https://techdocs.akamai.com/cloud-computing/docs/set-up-and-secure-a-compute-instance ) to use my purchased domain name.

Following are specs.

OS: Ubuntu 24.04
Host: Spring Boot Tomcat
Service: HTTP (no SSL, port 8080)
Cloudflare: Flexible SSL mode (regular HTTP to Linode origin host)

I can get the website pulled up via the Nanode's IP address (http://x.x.x.x:8080), and everything works as normal. When I try the domain name, though, it gives me a 521 error (server down). I'm really not sure what I've done wrong, or what else I need to do to get this to work.

I've tried with and without activating a domain in the Manager UI (and adding Cloudflare's assigned domain servers). I have added A/AAAA records for the DNS records in Cloudflare. I can't seem to set a port number, but from what I've read, 80 or 8080 wouldn't matter in this regard. So long as I've got it set to flexible mode, it should still connect browser-to-Cloudflare via HTTPS and Cloudflare-to-host via HTTP. I don't know where to check logs to see if the connection attempts are being made and somehow blocked. Otherwise, as stated, I'm stumped. Any suggestions?