Do a factory reset on the TC to start with clear clean setup.
The Factory Reset universal
Unplug your TC/AE. Hold in reset. and power the TC/AE back on.. all without releasing reset and keep holding in for about 10sec. The time is not important.. it is the front LED rapid flashing that indicates you are in factory mode.
Release reset.
If it doesn’t flash rapidly you have released reset at some point and try again.
Be Gentle! Feel the switch click on. It has a positive feel.. add no more pressure after that.
TC/AE will reboot after a couple of minutes with default factory settings and will wipe out previous configurations.
No files are deleted on the hard disk.. No reset of the TC deletes files.. to do that you use erase from the airport utility.
Open your Airport utility and redo the setup of the TC.
It will probably try to take over any adsl system.. that is stupid and useless so do the setup manually.
I would recommend the following steps.
1. Do everything with ethernet if that is possible. The TC wan port plugged into the LAN port of the Sagem modem router. Try and use a different computer if your existing one has no ethernet.. or buy the thunderbolt to ethernet adapter which is useful and not over expensive.
2. Make sure ipv6 is set to link-local on both the ethernet and wireless on your computer. For example in wifi network prreferences open the advanced and go to TCP/IP tab.

3. Make sure you are getting an IP in the computer from the TC.. the router setting must show 10.0.1.1 after a reset.
Check the network preferences and the same screenshot as above will give you the details.
4. Open the airport utility and go to the top menu area. Open File / Configure Other.

5. Type in your IP address.. after a factory reset there is no password required. But if it complains it is public
Click ok.

6. You will then get the full edit screens where you can manually setup the TC to whatever you want.

7. I strongly recommend changing to all SMB compatible names, ie short 2-20 characters.
No spaces and Pure alphanumeric.
eg

8. Use same rules for wireless. Set security to WPA2 Personal..
use a password of 10-20 characters mixed case and numbers... but still pure alphanumeric.

You can use a different name for 5ghz if you want.

You can also set channels at least for 2.4ghz I am not sure if 5ghz is relevant for AC models.
9. Set up correct mode. Open the network tab. Set it to Off (bridge mode) in this case where the Sagem is the main router.
That makes the TC into a non-routing device.. WAN is allocated to LAN and
the TC is a dumb Wireless Access Point, Switch and network hard disk.

10 Make sure the Internet is set correctly.. for most people it will be dhcp.

You have now taken over the control of the TC and it should work just fine.