It was clear you didn't want to disable UPnP support on the entire network, but I couldn't tell whether you'd tried disabling it on the NAS.

Does the following disable the FS2-210's local UPnP?

Go to TOS Desktop> Control Panel> Network Services> Discovery Service> UPnP Discovery > Uncheck "Enable UPnP discovery service"

https://help.terra-master.com/TOS/view/?lang/en-us/flag/disc...

I assume this won't break anything you don't want broken (ie- automatic port forwards), but I'm with you that the option is needlessly ambiguous.