Windows 10 1607 LTSB cannot install security updates

I'm running Windows 10 1607 (build 14393.0) Long-term Service Branch and I want to install the latest updates on it (I'm only interested in security updates). When manually searching for updates using Settings > Update and Security > Windows Update nothing shows up. It says that there are no updates present and that there are no updates installed.

When I try to install the latest security update from the Windows Update Catalog (KB4343887, cumulative update x64) and manually install it, it says that this update is not applicable to your computer (which it clearly is). Microsoft support says that it has four main reasons for this error to appear:

1. The update is superseded (which it isn't, since it is the newest update)


2. The update is already installed (which it also isn't, since there are no updates installed on the computer


3. The wrong update for the architecture (the computer is 64 bit, just like the update I chose)


4. Missing prerequisite update (at first I thought that this was a possibility, but I chose the cumulative update, which contains all the previous updates)

When testing this with KB4103723 (may 2018) it didn't show this error. However, at the end of the wizard, it just said that it failed, nothing more.

Note: I know the support for Windows 10 1607 has partly halted and will be completely abandoned in October of 2018, but this is the LTSB version, which will be supported for a total of 10 years.

If anyone has had the same problem, please let me know if and how you fixed it.