Pro Max Tower T2 FCT2250, Windows 11 has no administrator/domain connection

Hello,

We recently received a new Dell PC with pre-installed Windows 11 Pro at our company. I installed it using the intended user's Microsoft account. That part worked fine. The PC is on our LAN and has DNS and DHCP. Internet access is working. However, the user doesn't have administrator rights, neither locally nor in the domain. When trying to change the user rights or join the computer to our domain, it prompts for a different user. The domain administrator account, which works on all other computers, is not accepted. I've already tried the workaround of using the Windows 11 troubleshooter and the command prompt to activate the local administrator account with the command: `net user administrator /active:yes`. The command executes successfully in the console, but the administrator account is deactivated again after restarting the PC, so logging in is impossible. Does anyone have any suggestions on how we can activate the local administrator account or make the Microsoft user account used for the installation the local administrator? I can't do it through Windows user settings; it's denied when I try to change the account or add the user to the local administrators group.

Thank you in advance and best regards.