There is no security monthly quality rollups for Windows server 2016, like we used to have for earlier windows OS versions (before win 2012).
You need to install SSU followed by CU for each month.
The servicing stack is the set of components that update the operating system. (Patcher)
The servicing stack update (SSU) patches the patcher.
Cumulative Updates (CUs) do the fixing or securing the operating system itself.
Microsoft strongly recommends you install the latest servicing stack update (SSU) for your operating system before installing the latest cumulative update (LCU)
Sept-2019 patches for Windows server 2016 for X86 systems
Link to download SSUs and CUs : https://www.catalog.update.microsoft.com
windows server 2016 servicing stack
windows server 2016 cumulative update