You may already be aware but allow me to provide a high-level overview which may or may not help.
I don’t think it was mentioned but when Realm became MongoDB Realm, the strategy for setting up and working directly with the database also changed. Previously you would access both local and Realm Cloud realms with Realm Studio. That App is now for local only.
Going forward (and once the data is migrated), you need to use the online MongoDB Console to set up your app and work directly with all of the Realm data. This is the login link.
Secondly, Realm is a local first database. That means that what is stored in the Realm Cloud may be a copy of the data that is stored locally (data is written locally first and then sync’d later). So you can use Realm Studio to access the local data.
However, legacy came in two flavors; Full Sync and Partial (or query) Sync. If that was set up as full sync, all of your data is local and what’s in the cloud is just a copy. If it was partial, you will need to go through a different process. You didn’t mention which is was.
If you need to use Realm Studio 5.0.3 and are running Catalina, you can run older versions of macOS (and apps) using VirtualBox.
Keep in mind that if you do open a prior Realm file with Realm Studio, it will force an upgrade to the file which means it won’t be compatible with an older SDK, so you end up updating that as well.
Loop us in if you have more questions.