How do I stop this stranger from MDM and admin on this new iPhone?

This reported ongoing abuse and the reported break-in is a matter for civil or criminal courts.

The sort of ongoing abuse you have reported usually does not go away simply by ignoring it, and—in my limited experience and training—usually only escalates.

These reported issues with your own security—compromised passwords, cellular trackers, implants, carrier compromises, break-ins, whatever—are best handled through victim’s rights organizations, and legal counsel, and courts.

As for remote administration and the installation of MDM profiles, those only get installed if the iPhone is itself supervised, if the iPhone and its passcode was compromised and physical access was gained, or if the carrier needs specific Wi-Fi or carrier-related settings, or if the adversary exceedingly well funded. They’re unfortunately also far down on the severity of what-all has been reported here, not the least of which are the reported break-ins.

The activities you report imply your adversary is seriously well funded (as the individual exploits used in the espionage-like tooling here cost one to two million dollars each, and multiple exploits are usually involved), with internal carrier and international connections, and with the ability to access and bribe ~everybody, so either they’re massively wealthy and with access to espionage-level tools and you and your counsel both have a problem, or it implies they’re lying. Wouldn’t be the first time an alleged abuser lied.

Talk to a lawyer. Better IT is not going to solve your reported problems here.

There is no such thing as admin on your phone. Only you can install a profile or your organization can install a MDM.