Copy or share RDS Custom Snapshot from one region or account to another one

I´ve been doing some research and tests in order to copy or share a snapshot of a RDS Custom SQL Server database, but to no avail.

I got the next error when trying to copy the snapshot with AWS Backup:

You can't copy a DB snapshot for an instance in RDS Custom.

It does not work for cross-region or cross-account backups.

And if I try to share a manual snapshot, I got the next error:

You can't modify a DB snapshot attribute for an instance in RDS Custom.

There's no way to share the CVE either, so I'm thinking that maybe I will need to create everything from scratch in the second region or second account, and then restore the SQL Server database with a native tool or something along the lines.

Please advise, thanks!