I received the UPS, and I am really disappointed, it doesn't work correctly.
The CS350 is recognized by the DS210j, but it can not handle it properly.
For example, the UPS won't turn off after the NAS goes into safe-mode. It remains powered by battery, but when the main power is back, the NAS won't wake up from safe-mode, it needs manual intervension, a restart by the power button.
So, there is no way to use this system remotely (installed in the attic or basement).
I think that this is a driver related issue. I also described the issue to the Synology team, they responded quickly, assuring me that they will look into this. I just hope, that this will not take more than 6 months.