There is really no "best" way to set things up, but the following is by far the simplest.....and it is the simplest networks that always seem to have the least amount of problems.
The AirPort Extreme will need to be reset back to factory default settings.
Then, connect an Ethernet cable from one of the Ethernet LAN <--> ports on the Actiontec modem/router to the WAN "O" port on the AirPort Extreme
Use the iPad Air for the setup
All that you really need to do is assign a device name that you want to use for the AirPort Extreme, a name for the wireless network, and a password that will be used for both the device and wireless network. You can use the same names that you used before if you wish.
The setup utility will configure everything for you automatically, so you don't have to worry about jargon like Bridge Mode, DNS or IP addresses.
If you don't really need two wireless networks, you might want to contact your provider to ask them how to turn off the wireless function on the Actiontec router.
The more difficult setup route would be to first find out......IF.....the Actiontec modem/router can be reconfigured to act as a simple modem, and also......IF......your provider will help you do this and support the setup. They will probably be a lot less than thrilled if you ask them about this.
Assuming that it is possible and supported, the AirPort Extreme would be setup the same way, except in this case the utility will automatically configure the AirPort Extreme as the router for your network.
