Windows Server using WSUS with no internet connected

Hi,

I have some Windows Server 2019 is no internet access and configed to get updates via WSUS by GPO. But I haven't seen it communicated with WSUS in schedule. I checked WindowsUdpateLog on it, it connected to Microsoft Update SLS with fail becuase I configed it is no internet access and it haven't connect to WSUS. Currently, I need to check updates by manually using sconfig. 

How to config GPO is correctly with WSUS if the Windows Server is no internet access?