It's possible to use your own storage if it's using the S3 protocol.
This can be your own Amazon S3 account, but could also be a 3rd party solution like Ceph or Minio.
Digital Ocean spaces are build with Ceph and can be used. Minio can be installed on regular servers or NAS like Synology.
Please be aware that the storage requires fast, reliable and secure connections. For this reason we recommend using Amazon S3 for the purpose.