If anyone is looking to upgrade the memory in an RR2312, I have successfully done so using a SK Hynix 4GB DDR4-2133 PC4-17000 ECC Memory HMA451R7AFR8N-TF DIMM, which is 1.2V. I suspect just about any unregistered ECC PC4-17000 will work, but I chose the Hynix because it's the closest to what's installed on the motherboard. Note than non-ECC memory definately won't work (I had one, so I tried it first), and I would not expect registered memory to work, either. You cannot normally combine types, and the onboard is apparently unregistered ECC. The DMI info shows it to be 1.2V DDR4-2133 and unregistered, but does not show if it's ECC or not.
Based on your post I went looking for memory. I'm not familiar with Hynix, but I found Crucial CT16G4RFD4213 16 GB on NewEgg for $52.95. They have 4, 8, 16, and 32GB versions. The 4 and 16 are around $52, but the other two are more expensive. I bought the 16 GB, popped it in and it worked fine. My system now has 18 GB of RAM.
It took some searching to find out how to tell how much RAM the system sees. It appears the only way to see this is to get to the Logs page, download the logs ZIP and look at the memory log.
This post was the only thing I could find on the net saying anything about how to upgrade the RAM in the RR 2312.
When I was searching for information I did see somewhere someone said Netgear used to recommend expansion memory, but they quit because some memory makers changed the specifications of memory modules and reuse the same numbers sometimes and it became a mess.
