Question about QNAP TS-664-8G

My device claimed that the 8GB was not expandable. After receiving it, however, I realised that there was a free slot. So I ordered an additional 8GB module and installed it. Now I have 16GB without any problems. 8GB is rather tight.

I didn't find a slot for a RAM expansion, but I didn't look on the back of the mainboard either, only HDDs and SSDs installed.