In the old forum there was a thread TS-x53d RAM Upgrade Informative Guide (TS-653D, TS-453D, TS-253D, TS-251D, TS-451D2) - QNAP NAS Community Forum
I’ve tried to find compatible RAM for TS-453D (it’s the same for TS-253D and TS-653D).
I’ve installed two pieces of Kingston KCP426SD8/16 (16 Gb each) and they works flawlessy. Now my NAS is faster. 31 Gb available.
Giacomo
Where were you able to buy it? I am struggling to find 2666 slots and I am unsure higher frequencies will work with TS-453D.
Faster RAM typically isn’t an issue. The RAM will only run at the speed the mainboard & CPU can handle.
But, this is QNAP and they can be a little picky, so it’s better to only use sticks that have been confirmed to work. 
I suggest you to visit TS-x53d RAM Upgrade Informative Guide (TS-653D, TS-453D, TS-253D, TS-251D, TS-451D2) - QNAP NAS Community Forum where you will found a lot of information about ram upgrade. Not all configurations are working, so read it carefully. Better if you buy a well-known configuration. Note that 2x16Gb is working, despite of CPU limits. The important things is that you buy the right ones. In my case, I’ve removed the 8Gb bank and replaced with two 16Gb for a total of 32Gb. It’s working flawlessy from more than a year, no problems at all.
G.
In case you can’t find models reported in the forum, the best you can do is finding different models that respects everything you can read in the forum thread and try it. Buy from someone that gives to you some days to test the ram and return if it doesn’t work.