Recently I needed to manually install a cumulative update. It’s actually a simple process. Firstly you need to find the KB for the update. This can be found by navigating to https://support.microsoft.com/en-us/help/4000825/windows-10-windows-server-2016-update-history and select the OS build which will show a list of all the updates.
Once you know the KB head over to https://www.catalog.update.microsoft.com/ and type in the KB. You will now be able to download it and manually install as required.