This question was answered by @russellvernon:

After changing the droplet name in the Digital Ocean Dashboard to a Fully Qualified Domain Name, the PTR records created themselves.

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I had this issue as well. The fix here worked exactly as stated. In fact the, PTR record is what did the trick completely. I know this because I had not set up a trusted certificate, or the SPF and DKIM DNS records yet. It surprised me because I thought it would take more than reverse lookup to fix the problem.

Hello,

DigitalOcean automatically configures the reverse dns entry (PTR) on their end. It will be the hostname you choose when you set up your dropplet. You can change/check this in the control panel by selecting your dropplet, then settings, then rename. As it says, changing the name there will update the PTR but not the hostname of the system, that is something you will need to do, instructions for that vary depending on the system you have installed.

Hope this helps! Let me know if you have any questions.

Regards,
Alex