Mad_cyclist

The pcie slot I was using on the Unraid server motherboard was doing weird stuff. I tested the adjacent slot and everything worked fine, 9.9 Gbps on iperf as expected!

Thank you about the in/out sockets reply! I managed to find similar information from anther UPS, so I'll just ignore these ports from now on.
 
About your power consumption concerns, my router+switch+unraid+synology combined pull between 70W and 130W. The estimated runtime at 130W is about 15 min, which is more than enough to turn off both servers (that is my primary goal, orderly and clean shutdown to avoid data loss). Regarding the battery health, I purchased a new battery along with the used hardware and replaced it right away.
 
I managed to setup NUT on Unraid so that the UPS can be shared on the network with other devices. Follow step 1 and 2 from this guide: 
 
then go to your synology in Control Panel under Hardware & Power > UPS. Check "Enable UPS support" and select "Synology UPS Server" in the UPS type dropdown list. Set your unraid server IP in the "Network UPS server IP" box and you should be all set.