UCEPROTECTL3 is often an entire range block, so it’s probably DO who are being blacklisted rather than you. However, there is no formal delisting process, so DO need to stop spam from ANY of the IPs in the blacklisted range, and keep the range clean for long enough for the IP to be automatically delisted. That’s not impossible, but it’s not easy and it’s definitely not quick. Even when it’s done, it’ll take a while for the IP’s reputations to recover enough that I’d consider them good enough to send mail through.
For you as a customer, that means you’re not going to be able to get your IP whitelisted. Assuming you’re not the problem (i.e. you KNOW you’re not sending spam through your server - be sure of this before you proceed), then the quickest fix is to migrate to a different IP that isn’t on any blacklists. I don’t know if DO can change the public IP of your droplet, but even if they can, I’d personally opt for a phased migration to a new droplet so you can manage the process yourself.