Try to disable the DHCP of the DLink Covr 1100 and connect the router LAN to LAN and let see how it will go.
Here are some threads that you may find helpful about wireless/WIFI extenders. It may be a different router and modem, but the method can also be used on your router to connect with the Sagemcom modem.
I bought a pair of DLink Covr 1100 and wish to extend the signal of my 5g router (Sagemcom Fast 5866T). I went through the setting wizard provided by DLink and had no luck connecting to the internet with my Covr. The Covr just kept flashing orange indicator light. Is there anything I could try to get it work?