Hi,
The devices tab in Azure AD indicates the devices that are registered with Azure AD. Computer Objects synced from on-premises do not show up here.
So, you would have to register your client device with Azure AD as shown here: https://learn.microsoft.com/en-us/azure/active-directory/user-help/user-help-join-device-on-network
If you are using Hybrid Azure AD join, then you just need to run
1) dsregcmd /leave on the command prompt
2) Restart the device
3) You can check the status by running dsregcmd /status