Greetings,
Thank you for asking.
May I ask if you double-checked for any of the possible firewall settings for your DO droplet? 
Furthermore, do not skip below step and kindly re-check if Cloudflare is allowed to connect to your origin host to as follows in the below article:
Nevertheless, Cloudflare IP addresses list can be found here:
Regarding Cloudflare proxy mode 
and possible issues/errors as like Cloudflare timeout or 521, I’d suggest reading the steps what to do in that case as written on my recent post from below: