To enable your assistant to also have access to your account, we need to setup an app authentication method.
If you replace your cell phone the Authenticator app will not transfer. You will need to contact IT to re-register your MFA and both you and your assistant will need to walk through the setup again.
Go to mysignins.microsoft.com and then click Security Info on the left:
Click on "Add method"
Select "Authenticator app" and click "Add"
Click “I want to use a different authenticator app”
Note: Click "I want to use a different authenticator app" even if using Microsoft Authenticator app.