I would remove any setup you made on the routers to set up DMZ host, port forwarding, ssh after making a backup / notes of how you have things configured.
- enable NAT Port Mapping Protocol (NAT-PMP) or Universal Plug and Play (UPnP) on the FiOS router.
- Set up the AirPort in Bridge Mode
- Set up iCloud on the Mac to have Back To My Mac enabled (you can turn it off later - but this lets you use Apple's diagnostic test to verify NAT-PMP/UPnP are working on your specific network topology and settings.
- Power down the mac, then the AirPort, then the router. If any have battery backups, consider powering them down too. Start up the router, wait 60 seconds or so, then the airport, wait 60, then the Mac.
At this point iCloud preferences will show an error condition if you don't have inbound access to the Mac.
At this point you can go off Apple's Help in Finder - search for "Share your screen using Back to My Mac" - it should show you detailed steps and troubleshooting if it doesn't work. - https://help.apple.com/machelp/mac/10.12/index.html?localePath=en.lproj#/mh15597
To test remotely, use another Mac:
- sign in to the same iCloud account on that mac
- open terminal
- Shell menu - new remote connection
- select ssh
- select the host that is remote
Observe the shell command like
ssh -p 22 remoteMac.442113223.members.btmm.icloud.com.
If you can ssh in, then you know the router and the mac are set up to pass incoming ssh traffic on the default port 22 to the Mac connected to your router.
At that point, you can then think about making up tunnels or know if you are set up for IPv4 or IPv6 on the FIOS connection and routers.