I have been racking my head on this for weeks. Currently in 1802 with all hotfixes installed. Followed dozens of guides to enable the Pre-release "Enable third party updates on clients" feature. After everything is enabled per the guides, feature does not show up under Software Updates. This is a test lab build still on trial license. I built side by side production to test prior to implementation in production. The software update point component properties do not show any configuration items for Certificates either. Also WSUS is not within local computer certificate store but the cert is installed in Trusted Root Certificates and Personal..
