Restore Chat history - deleted Whatsapp - new Account (but, did Backups..)

Hi everyone,

My situation:
I got a new phone and wanted to migrate my Whatsapp History over. Thought I did it right and already deleted Whatsapp off my old phone, but f***ed up with the migration.
I did do the Whatsapp Google Drive Backup just before I deleted Whatsapp off the old phone, plus I downloaded the latest msgstore.db.crypt8 backup file from the Whatsapp Backup folder.

Problem:
My Whatsapp was registered under my old phone number thats long been disconnected. Now when I reinstall Whatsapp on my phone, I cannot get into my old account, because for that I need to receive Whatsapps 6-digit confirmation code on that old phone number, which obviously I can't.
If I create an account with my new number, Whatsapp does recognize the Google Drive Backup that I did earlier in the day (2GB), but fails to restore that, probably because I'm not logged in with the same phone number that the backup was originally created with.

I have about 4+ Years of messages that are very important to me - is there any way to fix this?

Thanks for any suggestions!!