QNAP TS-453D comes with only 4GB of RAM, and if you are going to do RAM intensive tasks like using Containers or Virtual Machines, you will definitely need to upgrade the RAM.
I bought the Crucial 8GB DDR4 2666 MT/s (PC4-21300) SODIMM 260-Pin Memory (CT8G4SFRA266) from Amazon.sg during Black Friday as they are having a sale. I got it for S$31.08 with free shipping to Singapore as my total order is over S$60 and I am an Amazon.sg Prime member.
Synology DDR4 RAM has been available in the market for a few years now and it is mainly targeted at businesses.
But with the release of the Synology DiskStation DS20 Plus series for consumers, you can finally get the DDR4 RAM to go along with your new shiny DS920+, DS720+, DS420+, or DS220+.