DD-WRT wireless extender

Setup -> Basic Setup -> WAN Connection Type -> Connection Type ->Disabled

Network Setup -> Router IP -> Local IP Address - choose an IP outside the DHCP Range from the main router
Gateway and Local DNS - usually the IP of the main router

Wireless -> Basic Settings -> First interface (2.4GHz) -> Wireless Mode: Client Bridge (Routed)
Default GW Mode: Manual
Gateway: IP of the main router
Wireless Security: same as on main router

Wireless -> Basic Settings -> Second interface (5GHz) -> Wireless Mode: AP
Set the WiFi network the way you want - You can duplicate the 5GHz config form the main router. this way the clients will do seamless roaming.

Services -> disable all
Security -> disable all
Access Restrictions -> disable all
NAT / QoS -> disable all

Administration -> Management
802.1x: Disable
Reset Button: Disable
Routing: Disable

Administration -> Keep Alive
Enable Watchdog: Enable
Interval (in seconds): 900
IP Addresses: Ip of the main router