Verizon will not support anything but the Actiontec router. The set top boxes work using the 192.168.1.100-150 range of IP addresses in the router and the port forwarding rules the boxes need are set up in the actiontec.
To answer your question what you have been advised to do is setting up your Cisco as an AP.
1. On your Actiontec check to see what range it is assigning DHCP in. If necessary limit/ adjust the range so you have a series of IPs you can use for static IPs. If your DHCP range is 192.168.1.100- 150 that is fine. Your Actiontec router is by default normally 192.168.1.1.
2. On your Actiontec you can either turn of or leave on the WiFi. If you leave it on select a channel 1-6-11.
3. On your Cisco turn off the DHCP server and assign it a static WAN IP. If your Actiontec is 192.168.1.1 you could make your Cisco 192.168.1.2. Set the 2.4 Ghz radio to a channel that is not the same as the channel on the Actiontec. I also recommend that you use a different SSID if you left the Actiontec's radio on. Set your 5 Ghz radio to whatever channel you like and give it a SSID that is different than any of the other SSIDs you have used. Connect the Cisco to the Actiontec as you said LAN to LAN. If you want to add another Cisco, use another channel, another SSID and connect LAN to LAN from eithet a port on the Actiontec or from the first Cisco.
Another option to connect either the first or second Cisco to your network is to use a MOCA adapter which will replace the Ethernet cable by using the coaxial cable that your TV is using. One of the real nice features of the Actiontec is the built in MOCA. Makes it very easy to install additional APs to improve WiFi coverage.
Bridging is another option but to keep it reasonably simple and since you want to keep your STB on your primary network for administraive purposes using your Cisco as an AP is a very good choice.
Some people on this forum swear at their Actiontecs, but I have no complaints about mine. The only time it has rebooted is when it downloaded a firmware update and I experienced a power outage that lasted longer than my UPS.
wan to lan means you have multiple subnets, and so you'd have to create a static route from one to the other.
Did you consider using a LAN-to-LAN connection?Instructions are here:»Verizon Online FiOS FAQ »Can I use my wireless or an extra router along with the Verizon provided router?A LAN-to-LAN connection will avoid double NAT'ing and the need to create a static route.