So, you can't access the following path on your Synology running Synology Directory Server?
\\
<syno-name>\sysvol\
<domain>\Policies\PolicyDefinitions,
e.g. in my case: \\
donau\sysvol\
ourhome.local\Policies\PolicyDefinitions
Guess, instead of syno-name, the IP-address of your Syno would work, too.
This looks like
Windows 10 ADMX build 1909, download:
Download Administrative Templates (.admx) for Windows 10 November 2019 Update (1909) from Official Microsoft Download Center
You will get a .msi installation file which you should run on you local Windows machine and copy the necessary files
AND language packs from e.g.
C:\Program Files (x86)\Microsoft Group Policy\Windows 10 November 2019 Update (1909)\PolicyDefinitions\ to the above mentioned path on your Synology.
Please choose the correct folder(s) with the language files, too and copy them (including the folder name(s), see hardcopy above) to your Syno, too.
To be sure, you could backup the SYSVOL folder before overwriting the files with new ones.
Usually, I uninstall those files from my local machine afterwards, by starting the installer again and choose "remove".
PS: if you need additionally ADMX files for e.g. Office (20xx), too, you have to add some more files and language packs.
HTH,
Michael
Policy Search Engine:
Group Policy Search