If your Mac is already in Apple Business Manager, but not showing up under MDM management, you may need to get a little nerdy to get it reinstalled.
First, using Launchpad or Spotlight, search for and launch the "Terminal" application.
In Terminal, paste or type the command below and hit return
sudo profiles renew -type enrollment
You'll be prompted for your computer password. Type that and hit return. Note that your password will not show up on the screen as you type.
You'll see a pop up message at the top right asking you to install or update your device enrollment profile. Choose update or install and follow the prompts.
