Hi Benjamin Fellner,
If a user is Global Administrator he can elevate his access to get all the subscriptions he need by changing the following option on Azure AD when they login.
Alternatively, you can provide on specific subscription access by navigating to Subscription> Access control > Add and provide Owner or Contributor
Please note: All roles mentioned above are High privillaged roles, assign them according to need.
Please Click "Accept as Answer" if this helps