Since you are connecting to the internet across Spectrum's transport network you should be using Spectrum's DNS servers which are located at 126.96.36.199 and 62. If your home network router is properly configured, the router will retrieve those addressses from the modem automatically each time it powers up. This happens for the Sagemcom or any other router you choose to purchase.
Other customers have told folks here on the forums that Spectrum has deliberately locked down many of the settings in the Sagemcom routers to ensure the devices operate as intended and do not 'flood' the network with excessive traffic.
If you want your data to be properly routed across Spectrum's transport backbone and out to the internet, you should be using their DNS servers. Once your traffic gets to one of the handful of IXP gateways, it uses an out-facing DNS at that location to determine routing beyond Spectrum's boundaries. OpenDNS knows nothing about DNS routing inside Spectrum, only after your data is out on the internet itself. You can program your personal router to use OpenDNS, but then you get to live with any extra delays.
DNS provides one simple service. It converts a "human friendly" name, like Google.com into an IP address, nothing more.
It has ZERO influence over how that packet is routed, because once the name has been resolved, it's the destination IP address that is put in the data packet that is transferred to the gateway assigned, and every single packet leaving your computer over a Spectrum cable connection will be routed to the same gateway on the Spectrum network. Now starting at that point and beyond, it's how the various routers are configured that determines the route the packet takes to it's destination, but DNS is NOT one of them.
If your rented modem is already set to "Bridging" mode you can simply remove the Sagemcom box completely and replace it with your Buffalo unit. You must enable WiFi, DHCP, and NAT in the Buffalo router. The ethernet cable goes from Modem LAN port to Buffalo WAN port. Wired device connections plug into the Buffalo LAN jacks. Never connect two routers in tandem - wierd stuff will happen even though you think you programmed it all correctly!