How can I remove Remote Management Credentials

You can try, but if the phone is under MDM then you need to contact the business that owns the phone.

• Open Settings
  • Tap General
  • Tap VPN & Device Management
  • Tap  any management profiles
  • Tap to remove the profile(s)

If you bought a used phone that’s under remote management, you bought a stolen phone.

Get your money back.