The QNAP TS-212p (item no. 2759368) is designed for 3.5'' hard disks according to the specifications. You would have to have a 2.5" to 3.5" converter like this one: ICY DOCK converter 2.5" to 3.5" inch, for SATA/SAS HDDs
But I can't confirm whether it really works.

In addition, NAS have a limited read/write rate anyway. So an SSD is not much use. So why not use a suitable NAS hard drive like the WD RED which is available from 1000 to 6000GB... WD Red (4000GB, 3.5", NAS)

Since my TS212, which is just over 2 years old, is already showing disk errors that apparently cannot be ignored or marked as unusable, the idea of using SSD "disks" instead of 24/7 rotating disks in a NAS was obvious to me. I'm sure the trend will be towards "NAS without moving parts"; maybe it's just too early now. These 2TB SSDs are still a bit expensive for a NAS solution.