To be clear, you are still connected to your old tenant? If yes, then i would assume that you don't have access to the subscriptions anymore since the permissions on those are set using IAM on those specific subscriptions and ressources, and by your explanation it sounds like you got entirely new users.
If i'm right, then you need a user with the Global Administrator role, when you have that, please navigate to "Azure Active Directory Services" --> Scroll down and find "properties" --> change the switch named "Access Management for Azure ressources" to yes.
Now go get a cup of coffee, this can take a few minutes. Please go to subscriptions and see if you can see all your "missing" subscriptions now. If you can, you need to set permissions for the required users for each of them again.
Please note: this changes your permissions to each management group and subscription to "user access administrator", so you are now allowed to set permissions on every level of subscription and management group