I am setting up a lab in my home for work and everything works great in the local network. I've added the appropriate static routes and devices on the 192.168.1.x network can communicate with everything on the 10.20.30.x network. However, nothing attached to the 10.20.30.x network can reach the Internet. I've attached a diagram showing how things are connected. I can successfully ping from a device on the 10.20.30.x network to the customer side of the FiOS router (72.x.y.4) but cannot ping the ISP side (72.x.y.1). I am guessing that the 10.20.30.x network is not being properly NAT'ed by the FiOS Router but I cannot seem to locate the NAT settings in the FiOS Router to make changes.
I run a similar setup on my network with my router double NATed behind my Actiontec router from FIOS with no problems.
That being said in a double NATed situation your Cisco router needs to be providing the private IPs for you devices connected to the CISCO router in the private IP range you have set.
1. Be sure that you are running the Ethernet cable from a lan port on your FIOS router to the WAN port of your CISCO router.
2. You have already given your CISCO router a WAN address in the subnet of your FIOS router so that is all good.
3. Finally be sure that the DHCP server is turned on on your CISCO and it is assigning addresses in the 10.20.30.0/24 range.
If it still isn't working try a different subnet on your CISCO such as 192.168.200.100 - 120.
With some router firmware there have been problems using subnets starting with 10. Shouldn't be that way but when things aren't working you need to eliminate all possible problems.
Going off of the comment above, is the Cisco or the ActionTec issuing out the Class A address space? Ideally the Cisco should be handing out those addresses so that NAT can do the job on the Cisco, if you intend for it to be that way.
Otherwise if the Cisco is set in a pass-through configuration, and the ActionTec is being used to hand out two different subnet ranges, then you might have to look at the default routing table for each subnet on the ActionTec. It's possible the Class A space doesn't have a route defined in the ActionTec, even though NAT may be configured.
Sadly, I ended up having to double NAT for this topology to work. I was trying to avoid having to double NAT and was very surprised that the FiOS router only NATs the attached subnet. Oh well - I can access everything, albeit not the most elegant solution.