Hi Rob,
Can you elaborate on which permission issue you encountered? If possible, please provide some screenshots.
To make you the admin, firstly log in to the environment with the account you used when you registered for the D365 trial.
Then when you are in the D365, click Advanced Settings > Security > User, can you successfully enter this page?
If so, select your account user, click MANAGE ROLES, then select System Administrator.
Generally, if you do not go through these steps to change your security role, then your default is System Administrator.
Therefore, the cause of the problem may be that you created a new user and used the new account to log in to D365 instead of the account you used to sign up for the trial version. Your original account is as follows.