I have been using my Synology NAS for a while, but the NAS recycle bin is always empty.
Upon digging, I found out it is all because of the File Service protocol I was using.
Apple Filing Protocol (AFP) does not support file deletion to the NAS recycle bin. It is an old protocol by Apple.
Server Message Block (SMB) on the other hand is by IBM and has been around since the 80s! It now has a version 3, so I suppose it has been upgraded constantly.
The important thing is – You should NEVER enable AFP.
Because anyone who connects to the NAS might accidentally delete a file, and there is no way to restore from the recycle bin.