SSL renewal for subdomain, main domain hosted elsewhere

Hello,

So for a few months now I’ve been trying to get Lets Encrypt to auto renew my domain.

I currently have a subdomain (sub.domain.com) - the A record is pointed to Digital Ocean. The main domain (domain.com) has many other records pointing to different places, so I don’t really want to switch the nameservers to Digital Ocean to use the auto-renewal that Digital Ocean supply.

I have managed to get an SSL running following various tutorials, however it would appear that Digital Ocean doesn’t see the renewals. I think I have to manually re-add the certificate each time.

Doing a dry run of certbot works, and I think my cronjob that checks once a day is working too - it seems to be generating new keys each time.

I’m pretty new to all this - is there a way to renew the same key and have Digital Ocean pick up on it, or using their “Bring your own certificate” solution do I need to manually do it each time?

Running Ubuntu 18.04 (LTS) x64

Thanks!