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.
Installation is straight forward. First, you need to remove all the drive bays to clear the space.
Next, reverse the RAM, and insert it at an angle. You will hear a snap sound when you push it down. This means it is locked into place.
Now my QNAP TS-453D has 12GB (4GB + 8GB) of RAM.