Couple of things
1) version does not use a modem. Quantum is a router
2) yes you can use another router in bridge mode. Generically it is a connection from LAN port of quantum to LAN port of other router. Disable dhcp on extender router.
No reason to disable quantum WiFi. For best performance, set ssid on both routers the same.
Ideally use an android app like WiFi analyzer and optimize channel config for each router to minimize interference and best performance.
