Whither macOS Server 5.3.6 installer?
Server is a separate app from the OS, that you download from the Mac App Store. It will be listed in the Purchases of the AppleID that was used to buy it.
You could also copy the app from your backup, and it will create all the necessary folders from scratch. (You may want to copy over the contents of /Library/Server, and other files, to restore your server settings.)