From 741 Doug and his Synology
I wanted to add a suggestion to troubleshoot Dougs issue with connecting to his Synology he stated was connected directly to the eero via ethernet. I too have a Double NAT as my ISP is not a Cable Company and they require PPPoE. The eero is not smart enough to work with PPPoE, which makes me regret ever paying for the eero to begin with.
With that said, I can connect to my Drobo 5N via my network. In my setup I run the ethernet from the main eero to a switch. As I am hardwired throughout my home my other two eeros are connected to ethernet. My Drobo sits in the closet connected to the network via the switch using ethernet.
Though I assume that the eero should have no problem seeing the Synolgy plugged directly into the eero's ethernet port it may be worth a try to use a switch and see if that corrects the issue.
The eero website states "The other Ethernet port can be used to hardwire another device (such as a Sonos player) directly to your network." Who knows, maybe with like PPPoE a NAS requires a different approach. Like I said I regret buying the eero.