Apple has a data privacy page that covers how to make requests for your data stored by Apple.
From there, request your other data:
Opening a case with apple after inspecting the other data might be needed - I can’t test your bucket as I don’t have that app or data stored against that container type / identifier.
Hopefully that bucket is not end to end encrypted, since Apple does not provide that data as a standard or listed non-standard request on the data request faq. your data seems to meet the other two requirements,
Any data that isn’t provided is either in a form that is not personally identifiable or linked to your Apple ID, is stored in an end-to-end encrypted format that Apple cannot decrypt, or is not stored by Apple at all.
I have emphasized the exception you may be facing. Also, have you ever made any backups of the phone? You might still have that data on the phone and be able to retrieve it from a physical / local backup since you will control encryption and/or have access to the keys if the backup does get (or was done) encrypted.
In your case if you sign in to iCloud on a new(ish) version of macOS you should get all the files synchronized to your home folder library.
cd ~/Library/Mobile\ Documents
If your application identifier is com.d3i.momento then it's data should be located in the folder iCloud~com~d3i~momento within the Mobile Documents portion of your library if the developer just stored apps within the app bundle. If they stored the data inside a group bundle, there will be letters and numbers before the folder like F6266T9T75~iCloud~com~d3i~momento