Since this UPS will be powering a NAS, I highly recommend you get one that can push out info to a NUT server via USB.
@ritchierich made an awesome driver that I use to monitor power outages and automatically shutdown my hub when the battery drops below a threshold.
QNAP NAS Integrations for UPS Monitoring, NodeRed, Home Bridge, etc
You simply need a UPS and NAS that supports the function. Your Synology probably does. A lot of Cyberpower and APC ups's support this function as well.