I've added a 16 GB module to my Synology DS920+. It all seems to work very well, but the Virtual Machine Manager still says I have only 4 GB available on my host. See attached image. Any advice on why this is happening? The main reason for upgrading, was to allocate more RAM to VM, so it is quite a bummer. I'm using Kingston DDR4 - 16 GB - SO DIMM 260-pin. Part number KCP426SD8/16. (Thank you very much for your advice on the unofficial RAM upgrade option! And all your advice on Nascompares in general!)
For others having the same problem: I had to uninstall VMM. This does not remove the VMs, so I did not lose any data. (And my continuously failing attempt to get the VMM export feature working was not needed.) When I re-installed, I had to add the store volume back, which was quite easy. Also some network setup adjustments. VMM then recognized all my 20 GB of RAM.
Possibly a bug in DSM 6.2.3-25426 Update 3, or in Virtual Machine Manager. Or something else.