Hopefully one of these can help point you in the right direction...
If your WiFi Extender only has an option for creating a new network, you could try setting this Extender's SSID / Password / Security to the exact same settings as the main router. This is typically how you setup a second Access Point in bridge mode when they are connected by LAN, so this may or may not work for you.
As mentioned by others, certain brands of routers and extenders just don't play well together, meaning their firmwares do not support certain configurations. I too had the Arris combo modem/router, but quickly replaced it with a separate modem & router once I found the Arris incompatible with many of Apple's AirPort features.
Someone mentioned WDS, which is an option if both devices support it, just keep in mind wireless extenders will typically cut the bandwidth in about half (since it has overhead from 2 way traffic).
Some devices, like Apple's AirPort Expresses (not the Extremes or Time Capsules) support a "client mode", which lets the AP/extender join your main network as if it were another wireless client. Other manufacturers (like Netgear) use a different name than "client mode", so you would need to look that up. The problem with client mode, however, is that some extenders (like the AirPort Expresses) will connect to the network wirelessly, but only pass the network on through its LAN port(s).
All AirPort Devices offer a better way of extending a network wirelessly using Apple's own proprietary protocol (WDS has been outdated completely by a better method on newer models). The catch is both the main router and extender have to be any of Apple's AirPort models. If not, you can still extend the network (without having to create a new SSID), you just have to connect the 1 Apple device and 1 other Brand device using a wired LAN connection.