In case if you configured a new WSUS server, but your Windows Server 2016 servers without internet access are throwing errors like “We couldn’t connect to the update service. We’ll try again later, or you can check now. If it still doesn’t work, make sure you’re connected to the Internet.“, you can fix it by following this ServerFault answer. It worked for me.
You probably don’t need to install KB4103720 and KB4462928 if you are looking at this post in 2020 or later, as it should be already installed.