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