Hi @Ben_Katz,
As a FIOS user, you should have what’s called an Optical Network Terminal that functions like a modem. It’s described here: Optical Network Terminal (ONT) | Verizon Phone Support.
Do you use FIOS TV at all? Otherwise, the ONT functions like a router, and you can hardwire your Core directly there to remove the double NAT layer that’s likely interfering with UPnP. See this thread: (ARC) FiOS users, You don't need the G3100 in your network (unless you have FiOS TV)
On a separate note: try disabling UPnP in the Verizon FIOS router now that you’ve created a manual rule.
We’ll keep an eye out for your response.