To have existing devices in your On Premise domain to join Azure AD in this hybrid setup, go back to your Azure AD connect server and run the Azure AD connect console.
Select “Configure Device options”
Connect to Azure AD using the account with the global Administrator role.
I only have windows 10 devices in my test environment, so I will only check the top box here.
Check the checkbox for your on Premise forest, and select the Authentication service and “Add”the SCP configuration.
Your devices should register to Azure AD after a few moment. However, in my case, because I had Windows 10 1709 running, it could not register with Azure AD. I needed to upgrade my windows 10 device to Windows 10 2004. Windows 10 1809 is the minimum for devices to join to Azure AD.