For FIOS app to work as a remote, you have to have your app on the same subnet as the STBs.
What can happen when people put in additional routers is that each router creates its own subnet.
So you need to modify your setup so that regardless of what rotuer you connect to, you are on the same subnet as the STBs.
Generically the way to do that is to connect your feed back to the FIOS router to a lan port on they additional rotuer.
Turn off DHCP in that router.
Should put it on the same subnet.
But as router models are sometimes different, YMMV.