Thanks for the reply. The reason why I asked about using the .htaccess was because that was one of the options suggested by Cloudflare, and I just wanted to make sure.
Quote: “You can explicitly allow these IP addresses with a .htaccess file or by using iptables.”
https://developers.cloudflare.com/fundamentals/get-started/setup/allow-cloudflare-ip-addresses/
You are right about the DO firewall. One of the reasons why I didn’t touch it.
Also, are we sure that the 521 error is most likely because of the firewall? Just want to make extra sure.