How do I connect an external hard drive to a FIOS router?

Help_Needed14

I have Verizon's FIOS internet service, and I have an Apple AirPort Extreme wireless modem attached. I would like to add an external hard drive by using the USB port on the Apple AirPort Extreme modem so that I can save or retrieve files from any device while connected to the network; however when I connect the hard drive, I cannot see it attached to the network. Any ideas on how to properly install the drive so that I can use it?

Edg1

My guess is because the Airport Extreme and FiOS router are on different subnets and both acting as a DHCP server. For example the Verizon router IP is 192.168.1.1 and Airport is 192.168.2.1.
Google setting up the Airport in bridge mode. Once it is configured in bridge mode they will be on the same subnet.