OK, thanks for the additional information. This really helps.
Connect an Ethernet cable to one of the 4 LAN Ethernet ports on the back of the Sagem modem/router. It does not matter which one of these that you use, but if they are numbered 1-4 and 1 is open, then it would make sense to use port 1.
Connect the other end of the Ethernet cable to the WAN "O" Ethernet port on the back of the AirPort Extreme.
Power up the AirPort Extreme and let it run a few minutes if it is not already on.
Hold in the reset button (located just to the right of the AC power cord socket) for 7-8 seconds and then release.
Allow a full minute for the AirPort Extreme to restart to a slow, blinking amber light status.
On the iPad, go to the Home Screen
Tap Settings
Tap Wi-Fi
Look for a listing of Set Up New AirPort Base Station
Just under that listing, tap on AirPort Extreme
The example below shows an AirPort Express as a new base station to be set up, but you will see AirPort Extreme on your screen.
When you tap on AirPort Extreme, that will start up the AirPort Utility setup "wizard". The wizard will take a few seconds to analyze the network and then display a screen that will look similar to the example below, except that you will see an AirPort Extreme pictured.
Network Name.......Type in a short simple name for the AirPort Extreme wireless network.
Base Station Name......Type in a short simple name that you want to call the AirPort Extreme device
Password.......Type in a password that will be used for both the wireless network and base station device
Verify......Type in the same password to confirm
Tap Next
The wizard will set up everything for you automatically. When you see the message of Setup Complete, tap Done
At this point, you should be able to select the AirPort Extreme wireless network that you created with your Wi-Fi devices and enter the password to connect.
Post back on your progress and if the AirPort Extreme is working well, then we will set up the AirPort Express.